Try Naxos or Paros. They have good beaches, Naxos has an interesting museum, and they are fairly easy to get to from Santorini. If you want something less touristed, you could try Sifnos, but it's a little harder to get to from Santorini than the others. Connections from all these back to Athens are good.

Suggestion: it's generally more effective to start one's own thread than to piggyback on an existing one.