Instead of water balloons make sponge bombs! This is a great way to beat the heat without using up a lot of water, and sponge bombs can be used and re-used all Summer long. (And don't have all those balloon pieces everywhere!
Ice painting is a fun way to cool off during the hot summer months. This website even has a substitution for children under the age of two who will want to put the ice paint in their mouths: kool-aid ice.
Colored water in squeeze bottles for drawing in the snow. Totally doing this this winter. Science and Art grade Teach a lesson on the weather changes outside. Then take the kids outside and paint the snow with these colored water bottles