So many good places to go, you probably can't go wrong.

Some suggestions of my favorite places in Spain:
1) Seville
2) Barcelona
3) Granada
4) Madrid

For some more information, you can go to http://www.lonelyplanet.com/destinations and look at Spain, which will have some good info for top destinations.

(Edited to add: Just saw that you said 'by the sea', but I'll leave my comments above intact in case you have time to venture away from the sea. Good luck! Oh, if you are in the south of Spain, you'll likely encounter the Andalucian accent, which may take a while to get used to.)