Take a 2L bottle of coke or pepsi, and open it. It's a little tricky, but take a mentos, and make a small hole in the middle of it, then string it on a string.

Then you put the string over the open bottle of pop, and close the cap on it, then cut off the excess string. Next person who opens it will have the mentos fall in the soft drink, and then a nice effect occurs

No fun to clean up though.