I saw this video of a cat on a treadmill [1] and I immediately thought that this would be perfect for our cat Tula [2] (aka (also known as) Miss Piggy [3], and not only because of her weight). Place her food dish at one end, and force her to exercise as she's eating.
[1] http://www.youtube.com/watch?v=bEQKotnQ2oo
[2] http://www.springdew.com/cookietin/main.php?g2_itemId=1476