Good Deeds is a super simple idea. You have write down an address where you can receive mail on a postcard and apply postage. Then perform a good deed for someone and write down what you did. Then you give the postcard to them. They do the same. When the postcard is full of good deeds, all the last person has to do is drop it in a mailbox and you get a list of cool good deeds you inspired.
The very talented VJ Brown put together a lovely set of four pre-done postcards layouts you can use to print your own cards to cut out and send into the world.