Hi samba,
I'm in Messinia, the southwest of the Peloponnese.
There are a few places that are small style resorts, but nowhere near as commercial as the main places on Rhodes of course! The only time anywhere gets busy is July/August, mostly Greek holidaymakers here, which to me says it all!
Ok, choices with the GF in mind, but geared a little towards your preferences too,
Tolon, Argolida (northeast Pelops) probably the biggest resort, liked it years ago, a bit too overdone now, although staying in the area would be good for seeing Nafplion (a bit more expensive staying there), Epidavros, Nemea, Corinth Canal, Ancient Corinth, Mycenae, Tyrins, & maybe a hop over to Hydra, Spetses &/or Poros.
Lakonia prefecture: In or around Monemvasia to explore that 'finger' of the Pelops, some lovely small coastal villages (Elia, Plitra, Archagelos, & one of my hidden away favourites, Gerakas Limenas), also possible to go over to Elafonisos & Kythira.
The Mani (the next 'finger' west): Gythio, a good size town, if you stay here make it towards the middle or the south of town. This is the top of the Mani on the east coast, the further south you go the more rugged & barren. The Maniots were/are a very strong race, & a little crazy (ever wondered where the word maniac comes from?), fascinating history. Go right to the south to cape Tenaro, back up the west coast, Gerolimenas, Areopoli, Stoupa (a resort) & neighbouring Agios Nikolaos, Kardamyli (this could suit you both), a small village with some little shops, the ancient village above it, some great tavernas, but not so great for the beach.
The westerly 'finger': Koroni, Finikounda, Methoni, Pylos, Gialova & Chrani might all suit, although the latter 2 have a good choice of a few tavernas each they have little else!
Just go to my photo stream on flickr & look at the collections
http://www.flickr.com/photos/yiassas/collections/ for 'lakonian road trip, "Local area' & Peloponnese. Not all of these places above are covered unfortunately, I only got this digital a couple of years ago

Well, that's a few places & sites for you to google & I haven't even covered the huge area of Arcadia, Ilea etc to the north & west, which includes Olympia! I have to say though that having a car & staying at a few different places is the way to go, although having a base for a week or so is good if it is in the right location for day trips.
Sorry about taking over your thread Angela, but it might give you & others ideas too!