Rosi and I completed the most funnest* science experiment since the one I did in high school biology that involved spitting into test tubes (I think it had something to do with pH testing, but I don't recall exactly).
We made Blue Goo. Seriously, this stuff is better than play dough. You can't sculpt anything out of it, but that's okay. It's fun just watching it melt and drip and change between liquid and solid form.
Do you want to get all gooey about science, too? It's easy, just mix one part cornstarch with 3 parts water. You can add a couple of drops of food coloring if you want. Mush it all together with your hands and let the fun begin.
Have older kids that are just too cool for goo? They can still play. My teenage niece turned me on to this one: Cornstarch Monsters. Add a stereo speaker to the mix and you can make the cornstarch colloid dance.
*Yes, I know it's ungrammatical. What's your point?