A small to medium sized knapsack or rucksack is also handy as carry on luggage and for day trips. In it you will keep your water bottle, your maps, print out of Harry's Greece Travel Guide, any snacks you want to carry along, a hat, a pullover, your camera, sunscreen, book, towel, swimsuit and anything else that you might need like medicine.
A waist pack is invaluable for just bopping around, with your stuff stowed away in your hotel (many have safes) or pension,. You can keep your passport, money, map and whatever in them so you can stop and smell the (Tri-an-da-fila) roses hands free.
I've tried jamming a camera in one and its uncomfortable. I recommend a camera strap and the smallest camera you can manage or a belt pouch.
You can buy waist packs and knapsacks in Greece all over the place but once you are on an island they cost more than in Athens. See the Athens Guide for shopping tips.