Well, if you want to make people think. You can challenge everybody to come up with the correct answer, and whoever gets closest win, or everybody can do it.
Take 20-30 mL of acetone, and ask them how many, or have them find out their selves how many of those really stupid and annoying Styrofoam peanuts that come in virtually every box that something is shipped in, will disolve in such little acetone. The answer is, a lot...like, a whole lot!
Another safe easy one you can do. You know those glow sticks you can get? Well, the intensity of the light is dependent on the temperature. So give people those, and a glass of cold water and a glass of hot water, and notice the difference. This is due to the kinetics of the reaction.
There are tons, it really depends on your "budget" and how safe/what you are allowed to do.