I will be flying into Athens with 3 friends and am looking for recommendations on which of the islands to visit. We are in our early 20s and will have 10 days to travel around. We would prefer using the ferries rather than flying.
Ios is the most affordable party island for 20-somethings. Check it out.
For orientation, here's a link to a map of the ferry routes, which will help decide which islands make sense in combination (if that makes sense):
http://www.greeceathensaegeaninfo.com/ferry-info-greece/greek-islands-ferry-chart.htm
As for where to go, it depends on your interests and budget. Los and Mykonos are known for being more of interest to 20-somethings and more "party" islands (more nightlife), some say Santorini is a bucket-list visit, other islands are slower paced.
You want to spend your last day in Athens so you are sure to get there without glitches for your departing flight, which leaves 9 nights (?) in the islands, so maybe 3 nights per is one idea...