One month in Spain - What city to live in?

We are thinking of going to Spain for our honeymoon in July, the whole month. Is there one place you'd recommend we set up base (rent an apartment), so that we could then take day trips around? I did this in Italy (one month in Florence) and it worked out great. I'm hoping we can have that same kind of spontaneous, exploring, laid-back experience in Spain. But where's the best base? Barcelona? Madrid? We would love a bit of history, a bit of beach, a bit of relaxation. Not into nightclub scene, really.