Brainstorm for Ideas
Planning and designing your tree house or playground structure is the most fun part of the process. Some great themes to start with can be the "castle" or "pirate ship". The only limit here is the limit of your imagination. Tailor the structure to your needs--if your kids love to swing, have lots of swings so friends don't have to wait for a turn. Consider lots of ladders, a fireman's pole or sandbox--whatever elements interest your kids.
Materials Basics
If you're using an existing tree in your backyard, make sure it is alive and sturdy. Use pressure treated lumber. Other good woods are cedar and redwood, but make sure you treat the lumber with sealant to ensure longevity. Use sturdy new hardware, never us anything old or rusty. You will need nuts, bolts, washers, quick-links, eye-bolts and 3/16 inch playground chain for swings, a drill and a saw. Use metal brackets to hold the frame together. Spread a padding material such as sand or mulch under swings and at the bottoms of slides to help prevent injury.
Enjoying and Maintaining Your Structure
Switch out swings as children age. Replace a baby-swing with a tire swing or a trapeze bar. When the kids move out, an older couple might enjoy a bench swing. Apply a new coat of sealant every year to extend the life-span of the structure. A well built and maintained structure will entertain the family for decades and give you the satisfaction that you made it yourself.