Greek adventure!

Hi Everyone!
My friend and I are travelling to Athens in October for the marathon. We were hoping to extend our stay another 5 days to explore some of the area. It's a little daunting with so many islands, so we need a little help. I'm sure we'll see enough of Athens while we are there, we are trying to sort out if we should do a greek cruise (if there's one that time of year anyone can recommend) or visit a few islands on our own. My friend wants to do Santorini.

I am interested in history, but really like poking around in shops for souvenirs, flea markets, shopping without breaking the bank etc. Finding places to experience the beauty of the area and keep busy shopping too would be ideal. Can anyone help???
Thanks so much!