Take things like sugar, salt, pepper, and dry herbs, garlic with you (depending on how much you are cooking at your accommodation. I find that those are the things that you end up spending extra on and it's such a waste as you hardly use any of it. I have found that almost everything is cheap in Spanish supermarkets.
By the way, it's too bad you can't bring loads of stuff back. There are some great deli shops that sell wine in litre bottles straight out of the barrel for 70p a litre!!