We just returned from a 2-week trip to the Greek Islands. We didn't have a set schedule. We traveled with backpacks, hopped on ferries and visited 8 islands:
The timing of this trip was crucial. We went in late May/early June. July and August would have been unbearably crowded. As it was, Mykonos, Delos and Santorini were already too crowded for our taste. Go too early in the season though and the ferries are not fully functional.
The euro traded between $1.08 and $1.13. We used ATMs to get our Euros but did carry US dollars with us. We were afraid Greece was going to default on its loan to the ECB and there would be a banking crisis. Our fears did not materialize and we had no problems with ATMs.