I have seen a few people who use diet cola loaded with Mentos to create fountains on Mythbusters.  Talk about an outdoor project! 

Basically we're talking about nucleation sites on the Mentos.  You will get much faster nucleation with steel wool, and a considerably bigger mess.  Activated charcoal would be faster yet, and considerably messier.

Please, before you proceed with anything so messy, file an Enviornemtal Impact Statement and warn the neighbors that you are about to create another flood!