Both Merida and Caceres are each worth at least a full day. It's also worth spending an hour or so up at the Roman reservoir looking at the exhibits on the way out from Merida. The reservoir was so well built it remains in use today.
Merida is full of Roman ruins in good shape, with artifacts. I did not come across many tourists, only a school group and a couple of pilgrims. I learned there at the Roman amphitheater for the first time that there were many different types of gladiators, each with a different kind of armor and weapons. They looked like nothing I imagined, certainly not like any movie depiction of them. They looked more fantastic than any sci fi characters. This knowledge came in handy later when I happened to be in Riolobos, a small village on the VDLP, on what I think was their Saint's feast day. The inhabitants there consider themselves direct descendants of the Romans. They therefore reenacted Roman things. There was a gladiators' contest in the park, with a dozen or so contestants. They were all dressed up as gladiators, in authentic gladiator costumes, just as depicted in the amphitheater in Merida. It was a truly fantastic sight. They engaged in half real combat using gladiator weapons, such as a hook and a net. They were so into it that a few suffered minor injuries drawing blood. There were several rounds. The event was well attended. All the locals in attendance were dressed in Roman costumes. They all knew each other. There was no tourist. It seemed I was the only non-local person in attendance. I chanced upon it because I wanted to walk around to see the village after my pilgrim chores were done. My fellow pilgrims did not attend but stayed in the hostel. After the final round when the champion was declared, he was hoisted by other men on their shoulders and there was a parade. Then there was a feast. I would not have understood the re-enactment so well had I not visited the Roman amphitheater in Merida and studied all the exhibits. What an experience!
Carceres is also memorable. While there I was out early in the morning to sightsee. I walked by the bishop's palace (Cáceres Episcopal Palace) which is not open to visitors. The gate was open so I tried to peek in. The bishop's personal secretary, who was there waiting for the bishop to come out to a car waiting to take him to some business, smiled and spoke to me. When he confirmed my pilgrim status, he allowed me to go inside the palace to take a quick look around. I got done before the bishop came out. After that, walking around the old town, visiting historical sights and museums, and engaging with locals completed the experience.