Water beads, aquagems, whatever you want to call them, are so much fun! The start off as these itty bitty hard spheres. But when you soak them in water, they grow to the size of a marble and are fairly firm and smooth (although I believe you can purchase them pre-hydrated). You can get them in a variety of colors. I gave Holly and Greta a small tub of them and a ice cube tray. Greta kept wanting to eat them, and while they're non-toxic and I still didn't want her to devour them. Holly loved them and calls them water "beans." Doing this activity taught me that we need a second toddler chair. Greta will sit and participate much better when she has a chair to help her stay in one place. We'll remedy that in next month's budget. Water beads are a winner in our house! 

water beads
water beads
water beads
water beads

Idea originally found here.