Dave, Mentos, and a 2 Liter Bottle of Diet Coke

Yeah, you know what's about to happen!

If you're interested in how we did this, you can check out this website that explains it pretty well, and tries to sell you the apparatus we have here, the Geyser Tube by Steve Spangler Science. I picked up ours at our local Hobby Lobby store and stuck it in Dave's Christmas stocking.
A Facebook friend suggested that you can do something similar by rolling up paper to hold the Mentos and holding a plain playing card underneath it. Remove the playing card and let the Mentos fall into the bottle. I like the apparatus I'm pitching above because it has a pull string that keeps you about a foot further away from the bottle, and you can get away from the shower of soda faster.