For a lot of people running should be fine for their knees if done properly.
As far as I can tell, running is most likely to damage your knees if you're (a) very big/heavy (b) have poor running technique (most people don't learn to run properly/efficiently) (c) run a lot on bad surfaces (avoid running extensively on surfaces that are banked, or where you may step in potholes!) (d) have a genetic predisposition to knee problems or have brought on ostearthritis-type conditions through poor diet (happens sometimes with exercise anorexics).
As a past competitive runner, I've spent a lot of time with running "lifers" (>10,000 miles on the legs) and knee problems don't seem to be particularly common (though obviously there are some selection effects there). Anecdotally, I have no knee problems after 6 years of 100mile/week training and most of my sports friends who do have them as a result of acute injuries (usually soccer).
That said, there's enough weak evidence to suggest that this kind of heavy aerobic training may not be good for long-term health and longevity to cause me to reduce my running to 20-30 mins/day (supplemented by weight training).
I actually did not want to go too deep into discussing specific sports and wait for another 24 hours, but...
I never had actual problems with the knees myself - I'm neather heavy, nor run that much at all (100mile/week for 6 years is extremely impressive!), also I eat helthy and think my technique to be okay. But I am very young. My grandfather, who has been doing a lot of sports his whole life (to my knowledge he still rides his bicycle for 50 miles a day or something at age 80) had some knee problems and therefore changed from relatively serious marathon ...
This post is a bit of an experiment; Most of the time, Discussion post lay out an idea and this idea then get commented upon. This post, on the other hand, will be purely about discussion on a topic. If this works out well, I'll might post more of these in the future.
On to the meat of this post:
I got this idea from a reddit post on /r/LessWrong.
To quote:
So have at it.
I only ask for one thing and that is to hold off on proposing solutions for 24 hours before giving suggestions for actual sports. In the first 24 hours, please discuss what makes current sports unappealing to rationalists and what aspects a sports designed for aspiring rationalists should have.
Edit: The 24-hour window has closed and solutions and suggestions can now be given.