hello!
if you are coming to Greece you shouldn't miss Santorini, that's for sure! To go by plane is quite expensive, especially during summer, but it can be at resonable prices during winter.
There are a lot of ferries going to Santorini, around 35-50 euros one way. In my opinion is better to take the ferry, there are more departures, it's cheaper and gives you the oportunity to see the other islands on the way.
Anthi