Barcelona is a fun city; I loved San Sebastian but in May it could be too cold to swim. The Pintxos in San Sebastian are great, much better than the tapas in the rest of Spain. Bar hopping to eat Pintxos and drink Sidra is great fun. Bilbao is a big crowded city. We went there to see the Guggenheim but I would skip both.
Sevilla has a nice cathedral and Cordoba has a very impressive mosque but neither are as entertaining as Barcelona.