A wood sandbox surrounding an above-ground pool can be like a stretch of beach. It will also afford children two places to play--the sandbox and the pool. Of course, you may consider the addition of stepping stones or a wooden walkway to cross the sand so that wet feet don't carry sand back into the pool. Because this is a specialty sandbox, whether or not you provide traditional corner seats is entirely a personal preference. The sandbox can be for play or just for looks.
Cut the 2 by 12-inch boards of lumber to the desired lengths to create the frame of the sandbox. If your above-ground pool is large, you may have to use more than one board for a side. Put these together end-to-end using a mending strap.
Make the frame using the 2-1/2 to 3-inch long nails to attach the boards at the corners.
Brace the corners using the 4 by 4-inch posts. Cut the 4 by 4 into four sections, approximately 11-1/2 inches tall so they sit inside the frame without sticking out over the top. Place one post at each corner and nail them into place, reinforcing the corner from the inside.
Cut the 3/- inch plywood into four triangular corners. Simply cut off the corners, making sure they are square. Match the edges along the corners of the sandbox and nail them down securely along the sides. This makes corner seats.
Fill the sandbox with sand.