Returning to Athens via ferry from Santorini

George Sifnos and Serifos are both good bets. Fairly low key, not many tourists, nice swimming and good food. Sifnos has a good bus system that makes it easy to get round the island. Serifos has only one bus but it does go to a spectacular hill town behind the port. Alan

Naxos and Paros are two good choices for that, as they're on the same ferry route as Santorini and Athens/Piraeus. From either one you can also take a daytrip excursion to Delos that includes a stopover on Mykonos for a couple of hours.

Thanks for your replies. The daytrips from Naxos or Paros sound interesting, but would it make more sense instead to stop there on the way back to Piraeus, rather than returning to Naxos or Paros? Also (and I'll check ferry schedules too), are these routes served by fast boats or the slower ferries?
Regards,

If you want to see Delos you have to do it from Naxos or Paros, then return to catch the ferry the rest of the way to Piraeus. There are some highspeeds available. Check here: http://www.gtp.gr

George, I was going to suggest Naxos, as I travelled that route in the other direction last year. It's one of the largest of the islands and would be a great stop for a few days. Happy travels!

Appreciate everyone's replies. Sounds like some good options. Of course, we're seeking to see and do different things than Santorini has to offer, but we're staying in a beautiful place there, and just wanted to ensure that we're not missing some variety by staying only on one of the islands for the entire time.
thanks!

Naxos provides a great contrast to Santorini. Many fewer tourists. Small port town. Nice beaches with very few people. Tavernas right on the beach. Your hotel can arrange the rental of a small car so you can explore the island and then turn it in when you depart at the ferry. Fantastic sunsets - take your camera.