If you weightlift, you probably already know this, but here goes. Doing less set/reps with higher weight builds muscle. Otherwise known as "maxing out". On the other hand, doing more sets/reps with less weight tones and defines muscles. This is what your looking to do.
If you like to bench press, dead-lift, or do anything with the bar, I'd probably do around 40% of your max weight. (If you don't know what your max weight is, I'd say something around 75-100lbs on your bar would be good. That's what I do at least).
Other than that, you can take the free weights (dumbbells) and go around the gym doing lunges, squats, etc. Maybe even run with small weights attached to your legs/arms?
Anyways, the point is, less weight, more reps and sets. Try running, increasing your flexibility, etc. Good luck and if you have any questions, you can PM me
As for covering stretch marks, maybe try some tattoo coverers? I know that Kat von D makes one and it works well from what I saw. Check it out here: http://www.youtube.com/watch?v=6qeIYMrBltY