I want to pick up a new skill in a day or two. Whether it be a new programming language, API, or something completely unrelated to technology, I'm open to ideas. What do you suggest?
Given, but he could at least learn a few basic steps, enough to get around the room. He'll also get the exposure to something new, which is most important (and probably what he's looking for).
Learn to surf. Or snowboard. You won't master either, of course. But standings up on your first wave or learning to link turns on the snow is exhilarating, and the first step towards getting better.
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):
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[ 4.7 ms ] story [ 33.8 ms ] threadOr how to fix something: a leaky faucet or toilet.
http://www.ehow.com/how_4563591_origami-bookmark.html
However, don't go overboard with the paper :-)
I also second the recommendation to ballroom dance.