Greek Islands - which one would you spend extra time on and why?

Hi all!
trying to plan my europe trip for summer 2011...
i have a spare 7-9 days allowed inbetween my two contiki trips.. was planning on spending some more time on the islands in this time once completing the athens to london + greek island hopping tour....
was thinking of spending an extra time on Ios? not sure where else though?

what are your thoughts and why?

thanks