One thing to consider is spending just two nights somewhere means probably 1 1/2 days as there's a lot involved with moving each time including packing/unpacking, settling in, not to mention driving time.
Nafplio is a wonderful town and the entire Argolis Peninsula is filled with just about anything one would want for the Peloponnese without the long driving time to various areas. You could spend your entire time for the Peloponnese there as a base.
The Mani is spectacular but driving time is very slow not because the roads are bad but due to the terrain: mountains with switchbacks, hair pin turns, constant stopping for photo ops, etc.
If the Mani is a must then I would split time between Nafplio and one location in the Mani. Areopolis is lovely and may work but I stayed in Gythio which is a lovely sea side town with enough services to keep you happy. You can do a day trip to Monemvasia but it's not worth an overnight stay in my opinion.
Eight days is only a drop in the bucket for the Peloponnese as it's a huge area so try to avoid to much time driving to get from point A to point B.