Having spent 4 days in Napflion, I looked at the tours offered. If I had to choose one (and you can quite pleasantly hang around town if you like instead), Mycenae would be at the top, with a close second of Epidavros. But we didn't go up to Corinth. It's definitely further away, although the distances aren't that long overall. We didn't make it to any of the forts either--there are two, one in Napflion, and one off-shore. Both of these could be done on your own, with a cab or a boat, respectively.

The Corinth/Mycenae combo looks attractive, if you're up for a long day.