Answer
1. April and May may be too cold for swimming and enjoying all that the islands have to offer. October could ba a bit rainy, so September or late August, and June or early july is best. Mid July to mid August is fine, but too crowded and expensive. 2. I wouldn't go on a cruise. I think you spend too much time on a boat, instead of visiting the life of the islands. If I were you, I would choose 2-3 islands and only go there, without a pre-organised group, perhaps they can arrange to book hotels for you. But you can do that over the internet too. Also you can book ferries, from www.greekferries.gr. 3. As for which islands to choose, we can help you if you say what sort of holiday you want. My favorites are Mykonos, Paros, Sifnos, Syros. We can tell you which ones you can combine because they are on teh same ferry line. Kos and Rhodos are beautiful too. 4. As for airfares, if you find something cheap to London or Paris or Berlin, you can then take Easyjet to Athens. If you book in advancce you can get cheap flights. 5. Visiting some mainland monuments, such as 2-3 days in Athens, 1 day at Delphi+Hosios Loukas, 1 day at Nafplio+Mycenae+Epidauros etc. is definitely a must if you have the time. 6. Obviously I don't know the tours you mentioned, because I am Greek. It is simple enough to just book your ferry and your hotel and just make your own schedule! Careful, some boats are slow though, check how long they take and consult us here at Y!Answers on whatever you need. 7. Have a great holiday!