The prescription medications are a must, of course, but you may also want to consider bringing over-the-counter medications in small amounts. In Spain you can find the equivalent of American medications, but if you're not feeling well, it's better to have those things on hand.
Also, the farmacias in Spain are not all open 24 hours like the American drug stores. There will be some open for emergencies, but you will have to hunt them down. Think of things you normally use, such as Advil, Pepto Bismol, Nyquil, etc. You don't have to bring loads of medicine, but a few essentials may be a nice safeguard.
Also, this isn't a necessity, but you may want to consider bringing a small notebook. I always bring a small, inexpensive notebook (I usually buy one for 99 cents at the local drug store before I leave!) so that I can write down where I went, what I ate, and other miscellaneous notes. I don't keep a journal because I'm too busy doing other things, but when I label my photos when I come home it's helpful to have the notes. I just keep it in my backpack with me at all times (with a pen, of course). I have kept all these little notebooks and it's fun to reread them.
Lastly, if you intend to send any postcards, you should bring addresses of your friends and family with you. You can buy stamps at any estanco.
Espero que te diviertas mucho, y tengo mucha envidia.