Miróbriga is the anchor that makes Santiago do Cacém worth a detour. The site began before Rome, then grew into a Roman town with forum, temples, baths, paved streets, and the remains of a circus used for races. It is open and exposed, so the ruins read clearly against the Alentejo landscape rather than being swallowed by a modern city.
Start at the interpretation centre if it is open, then walk the site slowly. The baths and forum are the easiest structures to understand; the circus is more subtle, but rare enough in Portugal to be worth seeking out.