Esmerlda - I went to Spain for MY 30th birthday a few months ago and here's what I did, I took my boyfriend along:
one night in Madrid, two nights in Toledo, took the AVE to Seville for two nights (I thought Seville would be the highlight of my trip, and it WAS nice, but I cut it short one night so I could spend an extra night back in Madrid!) Then back to Madrid for two more nights. We stayed near the Santa Ana area. The streets around Calle Victoria had great restaurants, a fun cuban bar with a band and nice nighlife. We didn't go to any wild discoteques but managed to stay out until 3 or 4AM every night and drank plenty of rioja wine. I felt like we had the perfect amount of time in each of the 3 cities. Spain is PERFECT for a 30th birthday!
I would recommend the Rick Steves guide to Spain and Portugal. It was the best guidebook I found for that particular trip. He has some good tapas restaurant ideas.
Good Luck!