If you have but one chance to see Northern Spain, the place to go is Santiaga de Compenstela. Her the remains of the Apostolate of St. James the Greater are found inside the second largest Basilica in Europe. And if the Anniversary of St. James, which is the 25th of July is on a Sunday, this will indicate a Holy Year and a spectacular celebration is put on here on this day. There are also celebrations here on this anniversary date every year also. The Basilica is the second largest outside of St. Peter's in Rome.
This is a little known tiny place called Os Ballotes in Vilagarcia de Arousa, a small town just above Cambados, which has a few rock engravings from around180-600 B.C. (Bronze age). It's a bit difficult to find the place, we were lost a few times, as there are no very good indications but the local people were ever so nice to give us directions. We expected to find a small place -- it was in a hilly area in a small community -- but not that small it seemed to be the private property or extension of the backyard of the house nearby. Until we saw the sign, it was not possible to know we had arrived. The drawings themselves, were no longer very well-preserved (there was no protecting roof or anything to protect it), and it was difficult to make out the figures. Still, it was worth seeing, and the little adventure of looking for it was fun.