An hour by train from Barcelona, Tarragona is a great destination for the history/archaeology buff, as it was one of the most important Roman settlements in the Iberian peninsula. After climbing up a steep hill to the center of town, you can visit the catacombs under an ancient chariot track, view the Mediterranean from the top of a Roman guard tower, and see a Christian church built in the center of a gladiatoral arena.
Added to the ancient sites are a beautiful cathedral and many nice plazas in which to enjoy a drink or a meal. Tarragona makes a nice day-trip from Barcelona, but a true fanatic of ancient Rome might want to stay a couple of days to visit all the archeological sites in and around town.
Charming, calm, relaxing, it is a good destination if you want to relax from your days in Barcelona!