When you have to sign a waiver upon entry, it's a good indication there's fun to be had at this bayside build-it-yourself paradise. Indeed, your five-year-old may salivate over the zip line -- which sends kids zooming along a wire from a raised platform, ending up in a sand pile below -- but she'll have to wait a year for that. Everything else is fair game, though. Kids can check out saws, glue, hammers and nails, tempera paints, and more from staff, and help add to the landscape of scrap-wood forts and towers, concrete tubes, abandoned pianos, scavenged boats, ropes, cargo nets, wooden poles, and old tires. The playground was launched in 1979, inspired by the 1931 observation of Danish landscape architect Carl Theodor Sorensen that kids would rather create their own realities of dirt and rubble than play in his conventional playgrounds. "Why not give them designated spaces?" he thought. The idea spread slowly around Europe, where more than a thousand such playgrounds now exist, according to AdventurePlaygrounds.hampshire.edu, but Berkeley's is one of a handful in America. Kids need sturdy shoes and clothes you don't care about messing up. Provided parents have a registration form on file, kids seven and up can be left to play by themselves for a $6 fee. It's free for accompanied children, and all under seven require adult supervision. The playground is open year-round on Saturdays and Sundays, 11 a.m. to 5 p.m. -- it closes for rain -- and every day from June 19 to August 18.