I was going to write this as a reply in a friend's journal, and then I decided it was worth writing in my own journal instead.
Most of us these days need to lose weight. And the way for us to lose weight is to get more exercise. And most of us don't get enough exercise because it's so hard to find a form of exercise that is a genuinely enjoyable way to spend time and doesn't require making a trip to some inconvenient location, which often takes more time than the actual exercise. Many of us are also limited by the fact that the really fun forms of exercise, the things that we'd really push to fit into our days for the fun and not just because we need the exercise, require a level of skill and physical coordinatation that we lack. Most high-entertainment-value exercises are sports involving other people, and to make them work, we need to find not just other people also interested in the same activity, but specifically other people who are at a similar level of competence at the sport so it can be worth their time and ours. Of course, this is all old news, so why am I babbling about it?
Several years ago, I read a short story, author and title forgotten, which proposed a good solution to this problem. A solution based on technology that was pretty much science fiction when I read it, but which would come a lot closer to being workable today; something that I think some geeks ought to get out and make happen, because we need it. Basically, the technology was a full-immersion VR system, with tactile as well as visual and audio. I don't remember the exact details of the setup in the story, but it centered on a suit that allowed vigorous physical activity and moderately violent interaction with the VR environment. You could run and jump, you could throw things, you could fight. It was combined with software that made you a virtual caveman, and to survive and prosper as a virtual caveman you had to do hard physical activity. The two keys were that the VR system was smart enough, and individualized enough, that the effective performance of your avatar was based on how good your effort was compared to what you were capable of, and the hardware was something that ordinary people could afford to have in their homes.
The story was about the effect on social interactions when multiple people were able to play in the same VR universe -- an aspect which would certainly help in making it worthwhile.
But think of it: If we could participate in a MMORPG from the privacy of our homes where we got a vigorous, individualized workout from the computer instead of just sitting on our butts while we did it, how many of us overweight geeks would be able to maintain that all-too-familiar geek physique? And between the people willing to pay good money for a good game experience and the people willing to pay good money for a good exercise program, I think there's boatloads of money to be made in the bargain.