Agree wholeheartedly with UK Tog, it can easily be done on your own. After our first visit there by Cruise ship, we have twice stayed in Monemvasia for a couple of days on our yearly land tour of The Peloponnese. If you're feeling energetic and are reasonably fit, then the climb up to the top of the old town offers great views. If you walk through to the end of the town by the Hotel Malvasia there is a bar that sell excellent fresh Lemonade.