My husband and I will be traveling to Greece for a 10 day trip at the end of May. We have already decided to spend 3-4 days in Santorini and would like to pick 1 other island to visit (we will spend 1 1/2 days in Athens-either in the beginning or at the end of the trip). We would love someplace less touristy with beautiful beaches and good food...and possibly cool architecture or old world charm. We were considering Crete, but keep hearing 3-4 days is just not enough time to make it worth the trip. Any suggestions??

The Peloponnese. You can rent a car and drive from Athens if you like. Napflion is a good base for the eastern part of the penninsula. It has plenty of old world charm, believe me. And lots of antiquities nearby.

check out Sifnos

AmyJill, I think you are right that your time is a little short to see Crete. Why not stop at Paros or Naxos which will be on your return route to Athens from Santorini? Or, as WWanderer suggested, go back to Athens and drive down to Naufplion - a wonderful spot to stay and visit local historical sites. Also, your best bet is to make sure you plan extra time in Athens at the end of your trip in case of delayed connections.

Well I think you have been given good advice from Wanderer and Parosblue. if you want an Island stay then Naxos or Paros are good companion Islands to Santorini. Much less expensive much more real Greek experience and more authentic.
If on the other hand you are looking for old world atmosphere and charm Nauplion certainly fits the bill. Here are pictures of Naxos and Nauplion. Either one is a good choice.
http://travel.webshots.com/album/570796960FtMsPF
http://travel.webshots.com/album/551991579dJbWvh