Peloponnisos

The Southeast Greek peninsula and one of the most historic areas in this country

If I was limited to see Peloponnisos peninsula only, I would visit the following (but remember that Peloponnisos is the most dense in interesting sites, along with Crete and the islands, so the following is a very rough guide).

NORTH-EAST (AROUND NAFPLIO)

Ancient Korinthos and the impressive Akrokorinthos castle,
Epidavros ancient site and theatre (near Ligourio village), of course.
Ancient Mycenae (Mykines in modern Greek) of course.
Ancient Tirintha (not far away from Mycenae).
Methana peninsula (a wild landscape full of volcanic petrifications, remnants from a volcano, quiet small poor villages, I like it very much but others don't).
Poros island (picturesque city, wooded and very near to the main land).
Hydra island (no cars allowed). An overcrowded very picturesque town with nice buildings from the 18th-19th centuries, international high society likes it too much.
Spetses island (nice landscapes and beaches, historic town like Hydra, fortunately not so "high class")
Nafplio is a relatively large but nice town with old buildings and three castles, a bit overcrowded of course (not as much as Hydra). It was the first capital of modern Greece.
Argos has a castle and a large ancient theatre.

NORTH (MOUNTAINS KILLINI AND CHELMOS)

Nemea (ancient excavated site, somewhere near the main road there is a nice small monastery on the rock).
Stimfalia lake (but do not expect too much water in the summer).
The villages on Chelmos mt. are nice (like Lousi).
Kalavryta (typical nice old small town of Peloponnisos) - Zachlorou village - Monastery of Mega Spilaion (large building on the rock). This is a nice trip. A nice alternative is to get on the special train (running on tooth-shaped lines) from Diakofto to Kalavrita, so you can also see all of the Vouraikos gorge.
There is a nice cave called "Spilaio ton Limnon" near Kastria village, visits are allowed.

NORTHWEST AND WEST

A must is, of course, the ancient Olympia.
Ancient Ilis (near Nea Ilida village).
Kyparissia is a nice small town with a castle above it.
Routes from Pyrgos to Tripoli are also nice (forests and villages). If you go there, you can visit the famous ancient temple of god Apollon near village Vasses.

CENTRE (ARKADIA)

This area was the financial centre of Peloponnisos during Turkish occupation (when pirates and thieves made sea sides unsafe). After the generation of the modern Greece, this area started to fade. There are many nice villages with stone-made houses, a usually poor but picturesque place.
Lagkadia village was the home of nearly all the masters who build most stone made mansions in the peninsula (except Mani area).
Dimitsana was also an important small town as long as Andritsena.
Near Dimitsana there are some very nice monasteries build on steep rocks (Moni Emialon, Moni Prodromou, Moni Filosofou).
Karitaina is my personal choice (a nice old village with a nice rock and a castle over it, many medieval-age churches etc).
Stemnitsa, Vitina and Leontari are also nice and historic villages.
East Arkadia (Aegean sea side) is called "Tsakonia" and it has nice villages like Platanos, Polidroso, Kastanitsa, Paralio Astros, Leonidio and the monastery called Moni Elonis.

SOUTH WEST (AROUND KALAMATA)

Ancient (3300 years old) "Nestor mansion" near Eglianos village at Pilos.
Neokastro and Paleokastro castles at Pilos.
Methoni (for it's well preserved castle only).
Koroni (castle and picturesque village).
Ancient Messini at village Mavromati (a huge ancient site).
Rintomo gorge, Nedousa, Alagonia and other villages on the road from Kalamata to Sparti through Taiyetos mountain.

SOUTH (TAIYETOS-MANI) AND SOUTH EAST (AROUND SPARTI)

Mani is the south Taiyetos area where people were never under Turkish occupation. They were organized in feuds, they were pirates and warriors and they were a very tough people in a very tough and infertile land. Their villages (many of them are abandoned now, a spectacularly wild and spare view) are of the nicest in Peloponnisos. There are many tower-houses ("pirgospita") used by Maniates (people in Mani) for defence when the families were fighting one another. Old houses and towers are made of stone, however they were not using any mortar at all. Some "pyrgospita" are restored and arranged as hospices, a very nice place to stay but it is a bit difficult to find a room in summer.
Some places to see in Mani (starting from Kalamata's side at north to south) are:
Zarnata castle and church in it (village Stavropigi), Tseria, Kardamili (tower-hospices), Itilo, Kelefas castle, Ano Karea (abandoned tower houses), Germa (ruins of a medieval city), Areopoli (tower-hospices), Pirgos Dirou (abandoned tower houses), Diros cave (maybe the best in Greece), Drialos, Mina, Stavri (tower-hospice), Vathia.
The road from Cape Tenaron to Githio is not as important but Githio itself is a nice old small town.
Near to Sparti is Mistras, a large medieval (byzantine) abandoned town with so many churches and mansions. You must not miss it in ANY case.
Relatively near Sparti there is the medieval castle town of Geraki (I have not being there until now) and some ancient sites (Amikles) and tombs (like the mycenean tomb at Vafio, famous for the golden masterly artefacts now exhibited in Athens National Archaelogical Museum).
If someone pass from this area, he must not miss castle-settlement of Monemvasia, one of the nicest places in Greece. However it is extremely difficult to find accommodation during tourist period (although maybe it worth a try).

KITHIRA ISLAND

Nice choice, a piece of "Greek island" colour, you can go there from Kalamata or (preferably) Githio or Neapoli. Chora (capital village) is nice, same for it's Venetian castle. There are many beaches in this island.

As you see I prefer ancient sites, castles, old monasteries, caves, dirt roads, wild landscapes and abandoned or desolated places.