There's a Christian Radio station in the Twin Cities called KTIS, and they have a thing they promote called "Drive Thru Difference." Here is the link:
http://www.ktis.fm/2010/06/drive-thru-difference-2/
Basically, you print out a note (or not, if you don't want), and when you go to get your coffee or whatever; hand the drive-through people the note, explaining what it is, and you pay for the person's meal/coffee behind you. It's a really great easy way to pay it forward. The note encourages the recipient to call in and on the air, tell what a difference it made in their day.
Just thought I'd share that! I love hearing the stories!