as you go down from Zagoria to Patras and then turning right to Athens, you go down along the Pindos Mountain Complex (from Ioannina to Mesologi) In the mountain complex of Pindos there are dozen sceneric villages that you can stop by for a night of so (if you are in for some more mountain scenery on top of the zagoria that you will have already seen). Also if you are into activities there several river activities such as kayak/rafting that you can do.
Ancient Olympia in not exactly on the route back to Athens, but a small detour that it is definetely worth the extra 100 (or so kms back and forth). about 8 years ago
From the places you have mention we can understand that you prefer the mountains instead of the sea side places, for this reason we suggest you on your way to Athens
at Antirio do not cross the bridge but tern to the way of Delphi. On Delphi you have the option to see the archaeological area and the museum of Delphi and then drive to Athens. This is about 1 - 2 hours driving more (excluding the time, you spent at the archaeological sites) almost 8 years ago