It looks o.k. to me! A couple of years ago we spent time in the Peloponnese. Three nights in both Athens and Napflion is a good amount of time. Monemvasia is probably a good stopping point though you have to park outside of the town. We spent a night there a did some hiking but otherwise, there's not much there. If I were doing it again I'd make it a daytrip but others may disagree.
Here are here are a few of suggestions based on our experience:
Pylos - we really enjoyed our stay and Karalis Beach Hotel. It's right on the coast and an easy walk to the main part of town. If you have a car, the nearby village of Gialova has some nice seaside restaurants. Also, if you like archeological sites, Nestor's Palace is fun to visit and if you like some light hiking, Voidokilia Beach is good.
Gythio - we stayed at a newish hotel called Las Hotel. It was modern and comfortable. There isn't too much to do in Gythio but it's a pleasant seaside town. We celebrated my husband's birthday at O! Live Restaurant and it was delicious.
You're going to have a great time exploring this region!