My favorite time to visit Greece is in the spring. Greece is both beautiful and quiet.

The weather is warm, but not yet hot. School has not let out, so crowds are light. And the locals will be very accommodating, as the tourist season has just begun. Also accommodations are both cheaper, and much easier to find.

With that said, I'm not sure it really makes much difference. The recession has severely impacted Greece, so crowds shouldn’t be too bad in May/June or in July. Santorini will be busy under either option, as it has become a required stop for every cruise ship in the Eastern Mediterranean. My recommendation would be to travel when it best fits your timing.

In terms of a third destination, that kind of depends on your interests. If you’re looking for the beach, really any of the other islands in the Cyclades would be a good choice. If you interests are antiquity, I would recommend either Mykonos (where you can visit the sacred island of Delos) or Samos (where you can take a ferry to Turkey and visit Ephsus).