We returned from Greece this past Sunday. Our itinerary was Santorini, Naxos, Nafplion, and Athens. We arrived in Santorini on May 13. At that time, temps were in the 70's and cooler at night and breezy. As we continued on with our trip, temps got hotter, and by the time we reached Nafplio, both Nafplio and then Athens were about 90ish. Very sunny. Very hot. My husband and I kept saying we would not want to be here during the summer months.
Regarding crowds, Santorini was crowded but I am sure it must be worse during the summer months. We stayed in Firostefani, which was less crowded than Fira and Oia. The big parking lot at Oia was a zoo with huge tour buses. Our private driver dropped us off and then picked us up later on when we called to say we were ready. I was fearful he wouldn't be able to find us in the packed parking lot. I was on the look-out for him!!!
Naxos and Nafplio and the Pelopponese Peninsula were much less crowded. Athens was very crowded, especially the Acropolis with huge tour groups. The Athens Flea Market was quiet, surprisingly, and shady. The Acropolis was very sunny and very hot.
What to keep in mind: wear good sturdy walking shoes/sneakers. Make sure you have sunglasses, hats, water, and sunscreen.