I really enjoyed playing with this little printer & the Zink App last year! (...or was it the year before?) I would go with this $199 ZINK hAppy Smart App Printer™ | ZINK®, and then you can use your larger iPad screen to design on.
Make some fun props for a "photo booth" (could just be a nice backdrop + volunteer with a camera + instant printer) at the event. Charge $5 per picture. This let's everyone take a fun memory home. It helps set a silly vibe and fun atmosphere at your event. :) Photo by propsonsticks on ebay.