Table Tennis:

It is a great way to burn calories, and gives me something to focus on/analyze and get better at. Using a body bug to track calories expended, I can burn ~4000 calories during a 2 hour session. It is a lot of fun, and you meet people from a LOT of different cultures. It is very international.

If you are in/near Des Moines, Iowa, check out http://dsmtt.org for the local details.

Also, check out http://pingskills.com for REALLY excellent videos that show you how to improve your game.