As a Greek person who regularly visits Greece, I 'd issue a "travel warning" of sorts. Don't go there this summer, the Olympics make everything more troublesome, crowded and pricey. Next year you'll probably be able to get the same stuff at half price.
Personally, I intend to stay well away till late September at least.
Ah, and if you insist on going but haven't booked your tix, try flying to SKG as opposed to ATH, as prices to the former haven't been THAT affected (much more efficient airport, too).
Haven't been to these two islands I'm afraid, I've heard mostly good stuff about Santorini, and that Mykonos is absolutely nothing special unless you want to show off. The only interesting thing about Mykonos seems to be its gay scene, but then again I am not gay so probably wouldn't be thrilled about it.
I could ask about specific properties if you want, but really do think twice before going to Greece this summer.