I studied abroad in Salamanca a long time ago and was in Madrid last December. I love Spain and I've been around the country a bit. Downtown in Madrid is very walkable and there are several days worth of stuff if you get a hotel near the Plaza Mayor. Toledo isn't far from Madrid and is super cool. The old Roman aqueduct in Segovia is amazing. If you go to Barcelona definitely see La Sacrada Familia. It is touristy but I spent a great evening eating and drinking outside on Las Ramblas. The Mediterranean coast is nice but there are lots of British and German tourists, which waters down the cultural experience a little. Granada and Seville are both gorgeous. If I were to go back I would travel along the north coast and then into the Pyrenees. It's off the beaten path and there are lots of cool fishing towns up there.
The best advice I can give is to eat and take your time doing it. Go to a tapas bar, grab a glass of red wine and eat whatever they recommend. Don't leave without getting some tortilla and cafe con leche. I'm happy to answer any questions that you have. I'm jealous.