What other activity could be fun to keep in shape for anything besides health for its own sake?

The only issue with cycling—and I say this as a committed cyclist myself—is that it's not really low-time-commitment. Realistically, an hour of strenuous riding will only burn maybe 500 calories (number direct from my power meter), whereas running will burn 150 calories per mile (based on http://www.runnersworld.com/article/0,7120,s6-242-304-311-8402-0,00.html, for 200lbs). At a 9-minute pace, it'll take you 30 minutes to burn those 500 calories: half the time. There's a lot to be said for cycling, particularly w/r/t cardiovascular benefits and low impact on the body (excepting crashing), and cycling is indeed awesome, but bikes are efficient, and that's a double-edged sword.

I am pretty sure there is no exercise modality that yields better results on a minute by minute basis than Tabata training. For a total time investment of about 45 minutes a week you can be remarkably fit. The bad news is, it will likely be the worst 45 minutes of your week. It is crazily intense. It is not boring, but I doubt you will think it is fun. Three 4 minutes sessions a week will yield you quite noticeable aerobic and anaerobic conditioning results. (the rest of the time is warmup and cooldown). Takes a lot less time than distance running, and offers anaerobic conditioning that most running workouts do not offer. As a side bonus, it is a great program for weight loss. (That being said, almost impossible to outexercise a cr@ppy diet)

Seconding Crossfit. Seconding a regular racquetball partner. Also martial arts. If it's a purely time thing, you might look into skipping rope. There's a ton of variation, it's practical, and you can pack a workout into fifteen minutes.

You can do relatively short bootcamp/crossfit style workouts that will be more varied than only running and build more strength. But if you have a community of runners on hand, it's worth trying just for the time with buddies.

A bunch of guys at my office play racquetball after work. It has the team/competition fix that working out lacks, but it's more flexible timewise than a league.

Well, I don't think that it is a time commitment thing - running enough to do half marathons is not exactly so easy that you can do it in a few minutes a day! But it is certainly more flexible in terms of being something you can do solo anytime you have the time. You know, not all organized sports have 'practices'. My dad loves sports and he is not much of a runner (he'll run with me if I ask him to but would never do it just for himself). He is 62. Here is what he does: - Tennis, he is in a number of tennis groups and if he doesn't have time to go in a given week, can ask other members of the group to cover him. - Racquetball - Soccer, he has played in an adult league, as far as I know they had no practices, just weekend games against other teams so it was more of a once a week thing. - He plays basketball in the mornings at the YMCA. They also have an adult league and it's just a pickup thing, you can show up when you want to, no practices or anything. - In summer he likes windsurfing, kayaking, and he recently took up kiteboarding - In winter he skis both downhill and cross-country, and snow shoes sometimes. If the snow doesn't come to us, we go to it!

Pickup basketball if you don't have time for a league? You'd have to find the right court/gym so you don't wind up with serious streetballers and get destroyed, but I play on weekends with what might charitably be called "mid-30s professionals" and might be uncharitably called "old dudes." (I'm an old dude) Do you have a gym by you that does casual classes in whatever? Mine has stuff like dance classes or lifting-focused workouts and you can just wander in if you're so inclined.

I'm in the same boat: always been very active in sports until life, job, and family commitments caught up with me. And, like you, I've always found running for the sake of running monotonous. I don't think you should just suck it up and start doing marathons or yoga simply because that's what your friends are doing. If those activities aren't things you actually enjoy, you'll resent any time you invest in them and probably end up quitting anyway. Ask yourself what activities you actually loved doing or have always wanted to try and then work those into your busy schedule if and when you can. As far as minimal time/maximum results, look into some type of http://en.wikipedia.org/wiki/High-intensity_interval_training. Over the years I've discovered that the thing I dislike most about running is that it's not competitive enough for me, the pace is tedious, and I'm really not fond of jostling around in a herd of people. The reasons I've always enjoyed soccer, baseball, tennis and sports like that is because they're fast-moving (you have to pay attention every second), there's a clear end goal in mind, and I get off on the mental stimulation involved in trying to outwit the other player or teams. Since I tend to carry a lot of pent up tension, I need competitive sports like that in my life to release it. Yoga, biking, and running are useful from a "zen" perspective, but if you actually need your sports to be more fast-paced and challenging, I don't think you'll be completely satisfied with those activities. Basically, I think you should do some type of HIIT for general health and weight maintenance plus whatever pick-up sports you actual enjoy and can work into your busy schedule.

Trail running might be more interesting than regular street running: you have to pay more attention to where your feet are landing.