I did fitness classes and weight training on my own for years, but about six months ago I discovered indoor (gym) bouldering and have made it my own. It's awesome. If you have a bouldering/climbing gym in your area, give it a try. I think a lot of people in the HN demographic would find it appealing.
- Easy to start: Bouldering is rock climbing but you don't need certifications, training, ropes, expensive equipment or belay partners. You stay lower to the ground than in roped climbing and fall on pads. If you have chalk (cheap, can usually borrow/rent) and shoes (rentable at gyms, cheap by most sports-equipment standards), you can go climb at a gym immediately.
- Goal-oriented, novelty-seeking, flow: instead of sets and reps, every bouldering problem is a creative physical and mental challenge. Good gyms tear down and set up new routes frequently and have a variety in difficulty, and most anyone can find something at the limit of their ability. Even if you're out of shape, if you can climb a stepladder, you'll have problems within your reach at most any gym (go slow to start though!)
- Instant feedback and progress: If you fall off the wall, you jump back on. If you go at least a couple times a week you can't help but get better.
- Motivation for fitness: The problem that's always just out of your reach at the gym will continually motivate you to drop a little bit of extra weight, improve your flexibility and get stronger.
- Friendly culture: Climbers tend to be a friendly, helpful bunch. No one cares who's better than who because the wall defeats everyone, and it's almost universally agreed that the most important part of the activity is having fun.