We spent a day in a half in Athens in May, it was basically enough to see the main sights.
I personally was very underwhelmed with Santorini. There are beautiful parts, but its also kind of dumpy and a humongous tourist trap. Some cool hotels, but the island itself is desolate and barren, though there are some cool beaches.
We went to Heraklion in Crete (the overnight ferry from Athens was nicer than I expected). Its a dense city, but seems fairly clean and not too bad to get around in. I thought Knossos was kind of lame. The actual ruins were ok, but they had recreated a lot of them with concrete and weird red paint, so it looked kind of like a parking garage.
I liked the mainland much better. Delphi and the area around it is awesome, its up in the mountains with gorgeous scenery. There's a little village just before you get there that's pretty cool. A bit of a tourist trap, but it was pretty quiet in May. Olympia was good,. The town was another shocking tourist trap. We stayed a few nights in Tolo, near Nafplion, which was pretty nice. Again, we were before the busy season, so it was pretty quiet. Mycenae was pretty cool. Nafplion was alright, the Palamidi Fortress was pretty neat. The Epidavros theatre was impressive. Corinth was good. We got to Acrocorinth and they had closed early, but it looked awesome from the outside (similar to Palamidi).