Have you ever microwaved Ivory soap? This is a neat little activity we tried over the summer. You must use IVORY soap...apparently this is the only type of soap that does this??? Place the soap in a glass bowl and microwave about 1 minute at a time. It should bubble up real nice, and create a snow like mixture for kids to play and mold with. The quicker you work with it the quicker it will mold. Cold soap will not form and mix, but if you place it into mold fast enough it should form. In all honesty, my kids could care less about the molds, they had fun smashing it, watching it "explode" in the microwave, and basically just crumbling it up. Overall it was a very fun "let's see what happens" activity.
Note: This is a messy activity...choose your space wisely...and your kids will smell like ivory soap all day...some of them will like it...others of them will demand a bath because them smell is too much for them to handle.