Day Trip to Tarragona, Spain

What and why to see Tarragona:

• Time Out: Was once the capital of half the Iberian peninsula and now contains the largest ensemble of ruins and Roman buildings in Spain - "an extraordinary legacy":  
• original town walls
• circus
• amphitheater (Amfiteatre Roma):  recalls the 2nd century days of games and gladiators
• aqueduct
• forum
•  Described as the grandest and most overlooked spot in Spain - Frommers, and it gives Tarragona 2 out of 3 stars.
•  12th century cathedral is a good example of the transition from Romanesque to Gothic.  There is a museum there with a nice collection of Catalan art. 
• Museu of Nacional Arqueologic:  collection of Roman relics and a very popular attraction - the mosaic Head of Medusa  -- closed on Mondays in the summer
• Museu i Necropolis Paleocristians:  one of the MOST important burial grounds in Spain used by Christians in the 3rd to 5th century

The Blue Guide highly recommends Tarragona for all the above reasons and has more extensive commentary.