15 pounds in 4 months? That sounds like a lot of time. Be sure to take it mathematically, Rye. There are two variables here that your actions control, and those are how much you take in, and how much you burn. So long as B/D (Burnt amount per day) is bigger than F/D (Fat taken in per day), you'll lose that weight. It's as simple as that, no challenge at all. Unless, like, you eat 10 Large Extra Pepperoni Pizzas per day or something. Then you're screwed for losing 15 pounds!

Anyway, a challenge I had for myself before was to beat every game represented by a character in Smash Bros. Brawl before its release, but I've given up on that, even though I only have 4 games left. Right now, I'm just concentrating on getting school done each day, and having fun.