Hi familyfive, the islands you have mentioned will be wonderful. I just want to add that we stayed several days on the island of Hydra. It's just an hour or so via passenger ferry from the port in Athens. There are no cars allowed on the island and the beaches are lovely. It's very quaint, quiet, clean and small enough that you feel like you can really relax and enjoy.

here's some info if you are interested:

http://www.youtube.com/watch?v=hYxbEHrcU8g

this was were we stayed but they don't have a pool (however, it's a short hop down to the town (which is good as no cars are allowed so walking everywhere is a given - unless you want to hire a donkey!) :)

http://www.hotelmistral.gr/

thanks nanabee, that island really looks lovely. I like the fact that no cars and the beachs are good. I will look into this.
We are also now considering Mykonos too. Santorini is booked in now.

First of all, I am Greek and I have been in most of our islands. Each island has its own unique character and atmosphere. I will recommend you 2 island hoppings:

The Typical Tourist: Mykonos-Paros-Santorini.
The Alternative Tourist: Astypalaia-Symi-Rhodes-Megisti.

I truly wish we had more time to explore more -thanks PetrosB3 for your suggestions

Petros' "Alternative Tourist" suggestion would make a good 3-4 week itinerary. Unfortunately most people who post on Fodor's don't have that much time.