Monemvasia is hard to get too, but well worth the trip. It is an authentic Middle Age Castle town, with a safety wall, cobbled streets and the original streets and building from the middle ages, but with all the modern comforts.
This place is romantic beyond compare. Absolutely beautiful.
Monemvasia is lovely old city/citadel placed on the big rock, connected to the land by causeway. You can spend the whole day exploring its narrow streets.
Alternative to walking around the citadel is exploring the coastline and finding secluded beaches just for two.