Weight sets are the most common thing that people buy and sell quickly. You can find a good weight set in the used section of the paper and save yourself a ton of cash. A good set of ankle weights to wear when you're not training will do more for you than you may think. There's a lot to be said for general resistence training with even a small set of weights to use to increase load.

Running stadium steps is one of the most intense ways to build a power jump from the blocks for sprints and interval training will help for the middle distance stuff.

I would also recommend a Tae Kwon Do class. Since it uses such intense kicking techniques, you will build up leg strength very quickly. Plus you get to learn something that may be useful to you at some time in your life.

Just a few thoughts.