If you do a carnival theme, you can enlist older children or other parents or family members to do face painting, nail painting, temporary tatoos, balloon animals if anyone can learn that quickly. Enlist others to help out while you wonder around to supervise. Hire a clown to come to do games and balloon animals. See if an older child or adult knows how to juggle and they can dress up like clowns. Get colored hair spray and do the kids hair funny. Get tons of those spray cans that shoot out silly string.
If your water pressure isn't good for sprinklers then just get several small pools and set up water stations with a slide going into a larger pool. You can use lots of food color to dye the water too, as long as they wear old clothes, lol. My older son done water balloon fights and water balloon baseball, where they just hit the balloons with a bat, plastic for the younger kids. Have Dad put on a fireman's hat and spray kids who are "on fire". How about a water balloon toss. Prizes to the winners! Beach balls floating around.
Pinatas are always fun.
How about pirates and mermaids? Treasure chests, water, boats, fish, lots of ideas there. Beach balls, fish nets, shells, eye patches, waist sashes and more!
Here's some sites for ideas:
http://www.marthastewart.com/article/carnival-theme-party...
http://familyfun.go.com/parties/birthday/feature/famf0803...
http://www.amazingmoms.com/htm/party.circus.htm
http://www.fun-theme-party-ideas.com/carnival-theme-party...
http://familyfun.go.com/parties/holiday/specialfeature/fi...
http://www.theme-party-palace.com/water-theme-party.html
K. B
mom to 5 including triplets
http://groups.yahoo.com/group/HarrisburgPAChat
chat and events within 2 hour radius