My kids (2 and 3) love to play with water and bubbles, and a great way for them to cool off and help out is to make a washing center outside.  We bring two large plastic underbed boxes outside, fill one with water and baby shampoo and the other with clean water.  They gather all their baby dolls, plastic toys and other water-safe items and wash them all with sponges and cloths.  They spend hours washing and rinsing, and toys left on towels in the sun dry quickly.   It's an easy, fun activity that will help keep their belongings clean too.