Origami! It's always fun and there are a ton of resources/guides/tutorials online. Most projects are usually short and practically all are doable in a day. Plus, many projects are cool to show off. Some are even useful! Here's a favorite of mine (simple, takes a couple minutes):

http://www.ehow.com/how_4563591_origami-bookmark.html

However, don't go overboard with the paper :-)