We got this crazy stuff called Oil Slick Slime. It's proper weird: liquid but not liquid. Gloopy, oozy goo that you can pick up and plonk back in the pot, but if you hold it in your hand it will dribble between your fingers... eventually. It's sold as a toy, but I think our kids are still a bit too young for it. And besides, it's far too much fun to waste on them!
It also makes a very good subject for lighting exercises, as it's so slick and reflective. This was lit by an SB600 through a brolly to camera left and a SB800 to camera right that was pointed at the ceiling, which gave the reflection on the top of the slick that defines its shape and texture. The SB800 also had its bounce card out to give the highlights on the leading edge of the slick.