These are a great way to connect with people who may benefit from your organization. And a chance to network with similar organizations at these events. Sometimes all that's needed is a signup sheet whereas other times one can get creative - treats, costumes, competitions, poster boards....the world is your oyster!
Pro Tip: Having paper signup is nice, but sometimes it takes a while/writing is messy so we don't reach people who signed up as quickly as possible. If you have a small netbook/laptop, asking people to sign up by typing their info can be more efficient.
Example signup form: https://drive.google.com/file/d/0B4UzW_kiS28hd1BWZkh5bFhsZE0/view?usp=sharing