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!
great summer idea-ice eggs - Freeze balloons filled with water and small toys. Cut balloon off and play with eggs outside. Provide spoons for cracking ice and digging treasures out. - looks like a great activity for hot summer days!