We've got some bargains through vrbo.com. Renting an apartment is so much cheaper than a hotel. Can you get a map of Europe and have a look at which places have big towns or cities and are next to the coast - that way you can have things to do if the weather isn't favourable or they fancy a day doing something other than lounging on the beach.
I rented an apartment through holidaylettings - it was a few years ago - and it cost about £600 for the week I think. It might be cheaper now, goodness knows the price of Spanish property has plummeted.
I didn't go, but you can get the train from Valencia to Port Aventura as well. Barcelona is also a lovely choice! The east coast of Spain generally has lots to offer. Flights from London to Valencia last year in summer were about £170 a person return, not bad for the height of the season.