Even if your kid never sits still, he'd probably still like a quiet, secluded place to hang out in your yard during the summer. If you have a booklover, how about a reading nook? A small clubhouse would be great for pretend play, and you can even make something specific, like a fort or a ship. Again, it doesn't have to be huge or ambitious. Even a tent will do -- teepee designs are especially popular, and you can buy or make them inexpensively. You can purchases plastic or wooden prebuilt playhouses, or you can assemble them yourself.
Parents who are really into DIY projects will discover a world of possibilities in books and Web sites on the subject. Safety should be your No. 1 priority, though. Premade structures or kits are designed to meet safety standards -- just follow the directions carefully for setup. If you want to build from scratch but aren't skilled in that area, enlist the help of people who are. Or consider taking a class. Decide whether you want a sturdier, more permanent structure or something that can be put away or moved. Some playhouses are built on concrete slabs just like real houses! As for outfitting the space, keep it simple. A basic house might just need a small cot or a table and chairs; a reading tent simply needs some comfy pillows. Make sure everything is outdoor-ready or easy to clean if there's a chance things might get wet or dirty.