Sevillas a nice city, one thing though it gets very, very hot there - so make sure u take an ample supply of sun tan lotion with ya.
Um, i went to spanish school for 2 years in Valencia (among other places) which is a great city on the Costa del Sol. Its relatively unknown to foreigners, even though it is Spains 3rd largest city. May be an option?
I would definately recommend going to the Costa del Sol if you are going. Barcelona would probably be your best bet, very nice city and quite a lot of foreigners. Only thing I would mention about Barcelona is the people that live in that area of Spain can be quite hostile to outsiders.