August is the worst time to be un-prepared. August 15th is the festival of the Virgin Mary (Greece's second biggest holiday, after Easter) and the worst weekend to travel in Greece; August 15th is analogous to the day before Thanksgiving in the USA.
However, the rest of the summer there are over 7,000 ferry trips scheduled monthly servicing the Greek island groups of the Saronic Gulf, Cyclades, Dodecanese, N.E. Aegean, Sporades and Ionian Islands as well as Crete, Turkey, Israel, and Italy and plenty of opportunities to reach your Greek island destination weather permitting.
What follows is some basic background information to help you prepare to go to the Greek islands and a few avenues of approach to consider as alternatives to the typical Mykonos - Santorini visit to the Greek islands.
I said that and I stand by it too! These two islands are among the best serviced by ferry of all Greek islands with several boats a day leaving for each from Athens area.
As for must see's you'd be surprised how many Athenians have never been to the Acropolis either much less Mykonos and Santorini so visiting these two is a personal choice. Both are wonderful islands if pricier and busier.
Give each 2 to 3 days and return to stay longer if you like them. See Santorini first if possible then Mykonos, even though the ferries from Athens do it in reverse. Start off in Crete (7 hrs from Santorini) to solve this problem.
Both Cycladic islands, Mykonos and Santorini are about 6 hrs journey apart on a typical Greek ferry. Mykonos is 6 hours from Piraeus and Santorini 11 by typical Greek Ferry. Less by the newer, faster, more expensive boats you'll read about below. You can also fly to these and several Greek islands but not all.
Ferry Costs, Ferry Schedule Chart, Note about FS, Ferry Operators, Piraeus Port, Going to Turkey, Greek Islands Map, Beaufort Wind Scale, C. Cyclades Routes, E. Cyclades Routes, N. Cyclades Routes, W. Cyclades Routes, Ferry Gallery 1, Ferry Gallery 2, Ferry Gallery 3, Ferry Gallery 4, Ferry Gallery 5.