Suck on ice or rub it on your body when you're overheated. This will help cool you down and prevent excess evaporation, which may lead to dehydration.
Eating a popsicle is a great way to restore water to your body as well. It's an easy way to get fluids into kids, too.
If you drink bottled water, freeze some in the bottom of an empty bottle, then top if off with cold water when you're ready to go. You'll have cold water ready to drink for hours. If you know you'll need more than one bottle of cold water, grab another full bottle, drain about an inch from the top and freeze the whole thing. By the time the first bottle is empty, you'll have plenty of cold water in the second.