I remember seeing this one pinterest when I was pregnant with my oldest son and thinking I'd try it someday. Apparently that day is today!
I filled a plastic shoe box half full of water and dropped some toys in. I put it in the freezer and when it was solid, I added more toys and more water. As an after thought I added a few drops of food coloring (bad idea, by the way).
When we were ready for the activity it was completely solid.
I asked my son if he wanted to free the little sea creatures and he took to the task with gusto.
Mommy had another idea...