Mycenae: In the second millennium BC, Mycenae was one of the major centres of Greek civilization, a military stronghold which dominated much of southern Greece. The period of Greek history from about 1600 BC to about 1100 BC is called Mycenaean in reference to Mycenae.
Palaia epidavros: Monestry, fortifications and open theatre from 4th Century BC
Delphi: Major ancient site.
Not been yet, going this year, these are the places someone originally from the Peloponese has recommended to us I had been looking at Athens but we have decided for several reasons to go to the northern part of the Peloponese.
Have a great time and do post back with your favourite place(s) after you have been! I am already enjoying reading Greek myths and legends to the kids at bedtime
Loads of things! Visit the new acropolis museum and also the acropolis itself. Walk from the acropolis round to the forum to the north. You can get the hydrofoil over the bay to Aegina which is a lovely little island, you can hire bicycles and head down the west coast to have lunch in one of the fishing villages. Walk up lycabettus hill for great views. Catch the early morning runners at the old stadium opposite the temple of Zeus. Wander the streets of Plaza at sunset but then go to Keramikos/Gazi (north west, walking distance) for dinner and maybe a cocktail. Great restaurant in Kolonaki called Oikeon (prounounced similarly to Ikea and means 'home' for some great food. There's a microbrewery somewhere in the centre too but I can never remember where
OP It depends how long you're there for. There is enough in Athens to keep you going for a couple of weeks. If you have time, as mentioned above Mycenae and Epidauros are gorgeous, but I would suggest staying in Nafplio and then visiting the other two. Nafplio is the more gorgeous town (first capital after independence). It really depends on how much time you have.
Sacha make sure you visit Nafplio when you go to the Peloponnese, it really is the prettiest town in mainland Greece.