[DonRotolo]

I have found that more than six mentos does not increase the reaction significantly (assumes 2 liters of diet soda)

Don't forget that non-diet soda is not as desireable, since the sugar damps the reaction and creates a sticky mess.

I'd stay away form the fancy stuff, like rockets that can hit (and hurt) you just as easily as go straight up, or leaving caps on.

Don