Go swimming. Organize a pool party, play pool games, dive for diving sticks, make up games with pool noodles, beach balls, etc.
Have a jumping contest off the diving board. Appoint someone or a parent the judge. Categories can include: funniest, highest, best dive, best star jump, most impressive, best 360 (turn), cannonball, etc.
Get a slip and slide or other sprinklers you can hook up to a water hose.
Go to the beach or lakeshore. Swim, bring a boogie board, fly kites. Make sand castles or sand sculptures.
Take a hike in the woods with friends. Go on a picnic. Go camping. Go bird watching, tadpole catching, pick wildflowers
Gather pretty rocks and stones. Paint patterns on the big ones, or paint them to look like pets.
Have a water balloon or water gun fight. Climb a tree or build a tree fort.
Go to a splash pad.
Get soap and various wands to have a bubble blowing contest. Try: most bubbles in a minute, biggest bubble, funniest shape bubble, most bubbles stuck together, etc. Blow up some balloons and play some games – keep it from touching the ground, juggle several at once, play volleyball. You can also decorate the balloons with markers.