Hi, here are just a few ideas:
get some kiddie pools and fill them with sand, have sand toys, build sand castles, etc
fill other kiddie pools with bubbles and have lots of different bubble toys (wands, etc)
hoola hoop contests, limbo, balloon toss, beach ball toss, frisbee toss
for food you could make the cute blue jello thing with the gummy fish in it (you can either make individual fish bowls or a big one), goldfish crackers, hawaiian punch.....
I might stay away from water guns at school. There is a great website for more ideas though:
www.birthdaypartyideas.com
(it's a very handy site) Good luck. maggie [img]smile.gif[/img]