Paros

c 1.25 hrs by charter boat from Mykonos
c 3.5 hrs by ferry from Santorini
c 4 hrs by ferry from Athens (or 40 mins domestic flight)

Paros, a short hop to the west of Naxos, is smaller and less dramatic, but does get busy in season (particularly the lively main port of Parikia).
The beaches are excellent, and it is probably the best island of all from which to do a little island hopping – every ferry in the Aegean seems to dock at one time or another!