Marvellous Mystras

Mystras was a functioning city that was abandoned in the 1830s and left largely intact. You can basically wander as you will through this vast relatively intact ruin of an ancient city. It climbs up quite a steep hill to a castle right at the top.
Most buildings have lost their roofs, but it gives you a much better feel for how those old cities would have looked than some of the more ruined ruins you find in Greece.
The climb to the top is definitely worth it.