I filled several balloons with all different things like macaroni, play dough, flour, sugar, pennies, salt, cotton, water and other things I can't remember. The kids played and played with them, squishing them, throwing them, stepping on them, and attempting to stack them.
When we were finished, I had my daughter guess what was in each one and then we cut them open. I was really surprised how many of them she guessed right. The favorite to open was the water. Both kids got a kick out of see how far they could squirt the water.