Mystras is awesome, I mean you're walking around ancient Sparta, when it became a medieval city.

Everywhere you go, the ruins look like the cover of a Black Sabbath album, which is one of the most incredible things ever. 

I've been twice, both times in the winter, once in 2002, and another time in 2017. There was hardly anyone else around, which also made it super cool. I don't know what it's like in the summer, but I always rent a car and drive around, and it seemed more remote on the Pelopenese Peninsula than most people would try to get to. I had a great time hiking around, but I'd also recommend exploring the general area, as people are friendly, the food is good, and there are great times to be had all around. Yasoo!