Wow, this is really cool! Have you ever heard of a Never Ending Card? I'll bet some of you have, but it's new to me. Check out this quick video to see a card in action:
How fun would that be for a greeting card, or invitation to a small party (because I'm guessing you don't want to make 50). The design of that particular card isn't exactly my style, but I'm sure you could make it hip and cool. Here are written instructions and also a how-to video. If you do one, let me know how it goes!