My coworkers are very supportive of my bike commuting, and one of them has taken it as an excuse to push everyone in the office to do something healthy. He's started running at lunch to match, and is suggesting that we lobby the nearest gym for a corporate membership. In fact, he told me that "cardio is great, but you need some strength training, too" and then mentioned ( as we all know ) that muscle mass boosts your metabolism, and burns calories even while you sleep.
I was going to say something about the heart being a pretty big muscle, and the bike working your legs ... but details aside, he's right. What other exercises do you enjoy? What do you think goes well with cycling?
In particular, I've heard a few people suggest something or other ( "core strengthening?" ) for better comfort on the bike. I'd be especially interested in this.