Have your kids played with waterbeads yet? They are really the neatest things. You can find them in the floral department of craftstores or even Walmart or the Dollar Store. Waterbeads start off as tiny seed-like balls and expand dramatically as they soak in water.
Kids love the feel of them in their hands, and lots of people have written about ideas for playing with them.
Teach Preschool has a very fun list with lots of inspiration.
For M and G's birthday we filled a large tub full of them, added some water and threw in several dozen glass and plastic marbles. Since it was a rainbow fish birthday theme we thought it would be fun to turn it into a fishing game. Each child took a turn with a blindfold and had a minute to pick out as many marbles as they could.
It was harder than it looked! The plastic marbles floated, and the glass marbles sunk; so contestants were free to find the method of sorting that worked best for them. Any waterbeads that ended up in the fishing bucket counted as negative points.
We had some competitive kids. They played over and over, beating each other's scores for over an hour and a half. (My poor husband was such a trooper in overseeing the game for that long!)
The next day, M, G, and I had bonus fun throwing the waterbeads at each other. We found them to be terrific ammunition as they are soft, bouncy, and wet.