If there's one birthday party spot guaranteed to please, it's the pool. Like skating rinks, pools are excellent venues because they often provide all of the entertainment you need. Perhaps bring along some extra foam noodles or water guns, and kids and their friends can usually amuse themselves for hours in a pool.
Meanwhile, you can set up cake, ice cream and food and clean up easily once they've jumped back in the water. In case you don't have a neighborhood or public pool to go to, don't forget about water amusement parks. Granted, these will be pricier than a run-of-the-mill pool party, but kids will have a ball.
Take age into consideration before going with a water-themed party, however. First and foremost, everyone should lather up with sunscreen lotion. Also, while older boys and girls should be fine, you may want to think twice about hosting one for younger tykes.