Balloon Rockets - I've done this in several science camps I've conducted. Kids have fun with it. But, when the balloon deflates it may fall off the straw because it's surface area changes - another good thing to show and explain to kids.
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!
Silky soft play dough for kids using 2 ingredients: 1 cup conditioner (cheaper the better), 2 cups cornflour/cornstarch, glitter (optional). I was just thinking about affordable gluten-free play dough.