Okay, hear me out. First thing you need is an exercise/yoga ball, big enough to sit on and use as a desk chair. My desk is kind short, so a 65cm one works, but they make bigger ones.
Sitting on a yoga ball is supposed to passively strengthen your core, which is why I decided to use one. But I started playing WoW, and found myself exercising! See, I play a spellcaster, so I have to stop and take a drink to regain mana. Well that's 15 seconds of idleness. So I reach from side to side to keep my back and arms from aching, or reach up (like the dancing tiger icon, but reach highighigh) to stretch, and it's a mini-workout. When I'm doing a lot of fighting, that's a lot of recovery time, and thus a lot of working out.
Then there's running around, or just autorun. Well, running gets boring, so you squeeze your butt together to make yourself bounce on the ball. Squeeze the glutes with each bounce, and you're having so much fun bouncing you don't realize it's exercise, even after you start to feel the burn! Or use your hips to rock the ball back and forth, and as you do that, crunch your abs and lower back alternately. Or see how wide you can roll the ball in a circle and try to keep your upper torso in place.
And you can do all the same things while crafting. Why, I come back with 35 copper to smelt, and I do one of these exercises while my character is hammering away, and wow! I can feel that!
You can do the same thing when you're at the computer pretty much any time. In fact, if you work at a desk, you might ask your boss about bringing in an exercise ball to replace your chair. I dunno if it'd work, but you can certainly try to convince them by touting the health benefits of the passive core strength boost.
Well I'm gonna get back to my workout now. ;)