Make-your-own-telescope station. Let crafty types decorate cardboard paper-towel tubes with stickers and glitter, then cover one end with blue cellophane (secured with a rubber band) so kids can see how things appear “underwater.
Go fish. Create a wave using poster board or a cardboard box (shown) and let kids fish for prizes. The person hiding behind the wave can use tape to attach the prize.
Peg-leg race. Divide kids into pairs and tie an ankle of each pair together with a bandanna, as in a three-legged race.
Walking the plank. Let kids take turns walking across a long wooden plank. Whoever finishes first without touching the ground wins. (If someone does touch it, yell “Shark bait!”)
Cannonball toss. Children stand in a circle and toss around a water balloon until it bursts. The person who gets wet is out, and the circle gradually gets smaller until there’s a winner.
Treasure hunt. Hide a bunch of plastic gold coins in the backyard and let partygoers race to find the most loot.