Recommendations for "cultural spots" to visit in Greece. On the mainland, there's Meteora and Delphi (north of Athens). On the Peloponnese there's Mycenae, Epidaurus, Nafplion, and Mistras (near Sparta). Islands with cultural gems include Rhodes and Crete.
For Greek islands that I've visited, I would avoid Mykonos and Santorini; they were wonderful back in the 1970s, but now have been horribly over-touristed. Generally avoid islands with airports and lots of cruise boats (two or three per day). Find gems that are quiet and off the beaten track--Sifnos, Tilos, Amorgos, Patmos.
My wife and I are going to Crete in April. We're avoiding touristy spots along the north coast, east of Iraklion--places like Malia and Agios Nikolaos, and are spending time in Chania, Rethimno, and Paleohora--in the west of Crete.
Hope you make it there. I love Greece. First went in 1973 and have been back another five times.