SLACKLINE Walking across a slackline—a piece of nylon webbing tensioned between two anchor points—demands balance and spatial awareness, both of which will come in handy when your kids take up skateboarding or surfing. Gibbon's Funline X13 ($80; gibbonslacklines.com) is kid-friendly—extra-wide, with grippy rubber print for traction.
TREEHOUSE Instead of purchasing a plastic prefab play structure, build your own. David and Jeanie Stiles's book Treehouses and Other Cool Stuff: 50 Projects You Can Build ($15; amazon.com) has some of the simplest, easiest-to-follow instructions we've seen.
IGLOO Our favorite winter toy is Grand Shelter's Icebox Igloo tool ($180; grandshelters.com), a lightweight, packable snow mold for building icy forts. The instructions are somewhat involved, but once you figure them out, you and the brood can construct a sturdy igloo in an afternoon. Extra credit: build a backyard ice rink. It's easier (and cheaper) than you think. All you need is level ground, PVC piping, and something to protect the grass. NiceRink sells fancier thermoformed side barriers ($32 each) and durable liners starting at eight cents a square foot (nicerink.com).