Went to Greece a few years ago in May before the summer crowds. Santorini was on the cool side and it did rain some but it was great to wander around and not deal with crowds. Do an e-bike tour of the island (https://www.santoriniadventures.gr/) . The views are amazing but I don't know if I'll go back but I'll stay here again: https://www.anandainfinity.com/ with a group
Naxos was wonderful as someone else mentioned. I'd go back there but am more interested in checking out other islands. There are so many! I only took 1 ferry ride and it was just ok.
I've never seen more graffiti in my life than in Athens but I don't mind cities so I enjoyed it more than my travel partner. Stayed in this tiny place but the layout, location, lighting, hottub, and view were all great. https://www.airbnb.com/rooms/22623197
Hope to get more recs from this thread for the next trip