A few weeks ago the I drove through Lexington with my family. While there we attempted to visit a coffee shop called "A Cup of Common Wealth" that had a cool theme. They really wanted to make it easy for folks to "pay it forward." Basically, what they did was put up a big cork board and patrons could buy a coffee, any size and type, and label what it is on the board via the cup sleeve. Then, when someone else came in that fit the sleeves identity they could take the sleeve down and cash it in for that specific drink.
Some of the sleeves are bought for specific people. For instance, one in the previously linked image, is for "Lori Benard" but they can be for groups of people or anyone really. So you could prepay for the most expensive coffee in the shop and label it - "Any Coffee for Anyone" and the next person who saw it could take it down and cash it in.
The coffee sleeve can have a condition - like "A Medium Coffee in exchange for Free Hugs for everyone in the room" - or it could have a very specific drink; "A free small vanilla Irish creme hot cocoa for a bearded man" you get the idea.
I really dig how they have taken the concept of "pay it forward" and made it into something concrete. Sadly, for us, the shop was closed for their annual staff Christmas party the day we visited so we couldn't see it in action or put a sleeve on the board.
One of our cool local coffee shops (bakeries) called "River and Rail Bakery" has a bit of a pay it forward implementation via a bucket near the register where you can plop in some cash. The limitation is a lot of folks don't carry cash. Via the prepaid option it would be a lot easier for regular customers to help someone out down the road.
The Cup of Common Wealth even lets you pay for a "pay it foward" sleeve via their website. Each sleeve you prepay for is $5 and you can put in the rules after you select the sleeve count. Then pay away. It seems to me that it is a good business idea and a nice way to build a culture around your shop.