Here is a plan to create a sandbox that'll keep the cats from using it for a litter box but will still allow some rain (or sprinklers) in to keep the sand from getting too dry. Dry sand easily becomes airborne and is a health hazard if inhaled.  The bottom is open and (if you decide to use the weed barrier) slow draining so it won't keep moisture permanently but will stay damp.  It's made out of cedar so it's weather, rot and insect resistant AND it doesn't off-gas and toxic fumes like the plastic boxes you can buy. If you just want to buy this and not make it yourself just click here and pick it up for $99 + $30 shipping. If you make it using this plan, it'll cost you (depending on wood prices in your area) approximately $140.

DO NOT USE ANY PRESSURE TREATED, WOLMANIZED, OR YELLA WOOD FOR A SANDBOX. Those products have formaldehyde and other nasty chemicals in them to stop wood rot and insects. If you're gonna make a sandbox out of those poisonous products, you might as well just give your kid a arsenic sandwich, blindfold him and tell him to go play in the street...

Since you're building a sandbox I'll assume you are a parent. Sign up for Amazon mom and get free 2 day shipping. The wife and I have been using it for almost a year now and it's awesome. 

Purchase List

Things you'll need to purchase for this project

• Box of 1 1/4 exterior grade wood screws. Long enough to bite well but not long enough to punch through 2 cedar boards and stab your kids
• Twelve 1 1/2 inch non-removable pin brass hinges
• Four 3/4x6x8 cedar boards to make the sides (you can use 3/4x12x8 but it costs almost twice as much)
• Six 3/4x4x8 cedar boards to make the slats for the top. Be sure to get nice straight smooth boards, as 4 of these will end up being seats or seat backs.
• Two 3/4x3x8 cedar boards to make the back braces and the seating braces/dividers. Make sure to get at least one really smooth board because it'll end up being the seating dividers that little bottoms will be sliding around on.
• One 2x4x4 board to re-enforce the corners.  You may only be able to find this in 8 foot legnth.  You can also use a 4x4x4 post to make it extra sturdy.
• 16 bags of playground sand
• Sandpaper
• Carpenter's pencil. This is a link so you know they look like, but buy it when you get the wood at your local store - prices online are silly
• Optional: weed barrier to keep the sand in and the weeds out.  My sandbox is on a slight hill that has a bit of erosion issues. 

Things you probably already have. If you don't, you should buy them immediately (cause they're fun)

• Cordless drill (bit driver). I HIGHLY recommend Hitachi drills. I got the combo deal and I use the regular drill for drilling holes and whatnot and the impact for everything else. I've literally sunk 6 inch 1/2 lag bolts into studs to hang plasma televisions with this tiny little drill. It's almost effortless with the impact mechanism.  The Li-ion batteries also last forever and charge back up really quickly.  You can get one of the bigger voltage sets,  but I've never needed it and the tiny little 12v is so light and agile I wouldn't get the 18v unless you're a professional and have need for much expanded run times.
• 1/8 drillbit for pre-drilling the cedar boards to prevent splitting, if you have a bit set this is probably in there
• Circular saw. I don't have a Hitachi saw, but I'd love one. I just don't really use it that much to justify the price. I use an old Skill saw I got at a pawn shop 20 years ago.
• Staple Gun. This is to anchor the optional weed matting. Get one like the one I linked, that uses T50 staples, the weaker ones suck pretty badly.
• Square. Awesome tool for cutting straight lines and for making joins as square as possible. Important tool for this project. 

Cutting

• Cut the 8 foot long 1x6s into 4 foot pieces. Please note that a 1x6x8 is actually 3/4x5.5x8.25. The way I cut my boards is I measured each one and halved it. Also keep in mind that your saw blade will take about 1/8 of wood off the board on the right side of the saw. In truth, this is pretty insignificant but do take the time to keep your halves even by measuring each board longways and halving it.
• Cut the 8 foot long 1x4s into 4 foot pieces. Halve these the same way as the 6s.
• Cut six 7.5 inch pieces out of the 1x3x8s and four 17 inch pieces.  The 7.5 inch pieces are the seat braces/dividers and the 17 inch pieces are the back braces.
• Cut four 11 inch pieces of the 2x4x8. These'll be the corner braces for the main box. You can also use a 4x4x8 to do this.

Assembly

Now all the wood is cut, the assembly can begin. You should strongly consider pilot holes for any screws near the ends of boards (all the top slats for sure). Cedar splits pretty easily; it's not very dense wood.

To make the base box, start by laying down your 11inch 2x4 pieces and squaring one of your 1x6x4 to it. Use a square to make sure it's true (or you'll end up with a wobbly un-square box). Run 2 screws into the 2x4 through the cedar board. Repeat for the other end of the board. Now add a second 1x6x4 to the top, and be sure to press these boards together firmly when anchoring the to board to the 2x4 to make a sand-tight seal.  Now complete this for another side. You now have two 1x12x4 sections with a 2x4 at each end. These will end up being the 'long' side'.  Set these upright, carefully square the remaining 1x6x4 to these sections you've created and drill 2 screws into each corner. You can see the 'short side' of the box in this picture. These 4 screws are run into the 2x end of the 2x4 piece. *Note - short side and long side refer to the fact that to make a box you need to overlap 2 of the 4 sides making a longer side where the width of the side board (3/4) is added to the length of that side.

Once you've finished up the main box you can put in the weed mat. I just stapled the mat up the board a few inches, rolled it out and stapled the other side. be sure to leave some slack to account for ground variation and the weight of the sand. The picture is of a few raised garden beds I made, but it's basically the same idea putting weed mat in the sandbox. 

To start the lid first lay out your 1x4x4 sections, trying to keep them paired for aesthetics sake. Rank your boards from smoothest to roughest. The roughest will be the outermost pairs (on the right in picture), the smoothest will be the 2nd pairs from the edge and the medium smooth boards will be the center. The smooth sides go DOWN towards the sand. Begin by squaring the boards to the box and drilling pilot holes with a 1/8 bit. Then screw the boards to the outside edge as shown in the picture. I left a 1/2 inch gap between the boards. Be careful if you're using a power drill here and DO NOT over tighten these screws, you'll split the board.

Now assemble the middle pair, the seat pair. Take the 1x3x8 and cut three 7.5 inch pieces for braces. Sand these down so they are nice and smooth. Use these to screw your smoothest boards together, making sure that the braces are flush with the long edges. Put the outer braces 2.5 inches from the end and put the 3rd brace dead center in the pair. You can see the layout in the first pics on this page and you can see a detail of the sanded braces on the left. 

To attach the seat pair to the outside pair, just lay it as it will end up when folded out but put a Carpenter's pencil in the gap. This will give you the perfect amount of space for the hinge. Secure the top hing side first, making sure to keep the pin exactly in the gap left by the pencil. I found it easiest to use a screw point to mark where I'd want the screws and carefully sink the first screw until it binds slightly. After that you can use the tension of the first screw to hold the hinge in place while you perfect the alignment of the remaining screws.  Now do the other side and flip it up and down to make sure it's all gonna work.  Next onto the inner pair...

 To install the hinges for the inner pair, we'll only use 1 board initially. You want to hold one of the inner most boards (the back of the seat, center of the lid) and screw the hinge into it as shown in the first picture (swinging opposite of the first set of hinges you installed). In the first pic, the smooth side is on the left. Do both hinges this way. Now, set this single hinged board on the flipped up seat. The seat braces will prevent you from being able to screw it in, but you can easily eyeball the hinge and put some marks where you'll want the screws to go (2nd picture). Now, pre-drill these holes to make it easier to set the screws while you're holding the board.  In the second picture you can see how I lined up the board and with the hinge hanging down eyeballed the spots where I wanted the screws to go.  The third picture shows you how it will look after you attach the single board to the seat.  We'll attach the 6th board using the back braces.

The next step is to attach the last slat using the seat-back braces.  These are the 17 inch 1x3s. Set them 1.5 inches into the board 2nd from the outside and square them. This will standardize the spaces in the very center and the two spaces nearest the center space to approximately 1 inch.

Rapid-fire backyard engineering... Once I had this all together and my kid started playing in it I realized that it had a design flaw and I'm not sure how to correct it.  The only way I can think to correct it is to use 2x4x8 for the top slats, but that still wouldn't do too much to fix the fact that cedar is just very pithy wood.  The flaw is that a child who is climbing from the sand to sit on the the seat will grab the top of the seat back to help pull themselves up. This pulls the seat back forward and places a lot of leveraged strain on the screws holding the hinge to the seat section. I'd say that, even if you used bigger, longer screws and somehow managed to not have the screw breach the surface of the board when mounting the hinge (a difficult task) it would still eventually rend the hinge. My solution was to throw together a quick wooden spinning latch that simply flips up over the seat-back brace and holds it snug so a kid can use it to climb.

I took the remaining wood from the 1x3 (~16 inches) and quartered it. Then I anchored one piece near the back brace with 4 screws and screwed a second piece into it pretty loosely. The to 'screw loosely' is drill the screw into the outer piece of wood and have it protrude a bit THEN sink it into the first piece. This prevents them from fitting super snugly and leaves a little spin room. Experiment with tension to get it just right. You can also put some grease on the head of this anchor screw to prevent it from unscrewing itself as you spin the latch. I placed a final screw on the top left corner to prevent the latch piece from spinning in circles and unscrewing itself. 

Put one of these on each side. If you have it tight enough one is all you'll need on each side.