Any activity that gets you moving and you enjoy is worth doing.
Rollerskating is just as good as any other form of exercise. It really depends on how intense you're working rather than what you're doing.
As far as the musclar issue.. Muscle helps burn even more calories so don't be so afraid of it. Unless you're genetically prone to getting bulky in the legs it's just not going to happen. Besides, musclar legs are a lot better than cottage cheese thighs.
To mefit, I was wondering...I try to do a lot of yoga as well. Do you know if that would 'count' as what you discussed as an adequate variable exercise, or should it be something more along the lines of walking, biking, etc.?