Here's my advice:
1. Poke around some secondary literature. Get a grasp of what kind of research paper you'd like to write
2. Go to a professor who you trust. Ideally, this professor has specializes in this time period/place so he/she can help you with an independent research project. If not, then he/she can "recommend" you to one of his/her colleagues. If so, then that colleague may be able to work with you.
I don't know about your university, but most professors left campus during the summertime. You may be SOL until next year. As another poster has mentioned, you can volunteer with a local archive, however, I don't believe this will help you very much. You can spend this summer studying for the GREs if your score is not high.
If you can write an excellent and original paper which incorporates primary source documents and foreign language sources, then I'd say you will be in good position for the next application cycle.