For me, the key thing keeping me on track is convenience - I mostly run and do DVDs, as per my signature. I always resented the time it took to drive/travel, change & shower to go to the gym - usually added at least 45 minutes to the whole thing. 2 hours of my day, every day, was more than I was willing to submit for the sake of my bum.
Mind you, I'm having now to read a bit more about fitness to try to keep things progressive (that's time too!) but hey. I do get bored sometimes... but the things I'd actually like to do (contemporary dance classes; dragon boat racing; skiing, in season) are kind of out of reach financially at the mo, but maybe soon that'll change...
I came across a website
linking personality (as defined by MBTI*) to particular activity types, but I guess most people work out their preferences naturally anyway. Mine all fit.
(I know MBTI is fuzzy, but in broad strokes, it makes a kind of sense...)