Sometimes I am asked by new sports shooters how to get access into MLB or NFL games. They seem to have the perception that that's where the best shots are. Granted, the action and skill at the professional is unmatched, but good photos can be found even with the little guys.
And the same basic principles apply...faces and some kind of action (preferably, with a ball in the image) are key. Of course, when shooting for sales to parents it's always good to get a few "stock"-type images. But one way to separate yourself from the next guy who just bought his first DSLR and lens is to capture peak action.
Hopefully this gives you someting to think about next time you head to the ballpark. Good luck and happy shooting!