Instead of push-ups and sit ups (because, A, most girls don't like push-ups and, B, most ppl do sit ups wrong and end up hurting their backs) if you want to build up a bit of muscle mass, I recommend simple things like leg raises.

Just lie flat on your back and keep your legs perfectly straight. Then raise your legs 6 inches off the floor; hold them there for thirty seconds. If you're feeling daring, lift them between 6 iches and 1 foot. Repeat that about 30 times.

We used to do those in taekwon-do long before people got on the damn pilates kick. Trust me - they are good for everything. (And without the fluttery hand motions the pilates ppl do, you won't look like a seal!)

-T. pirate