Donousa is the most northerly and most remote of the Small Cyclades, a small island off the east coast of Naxos with a single harbour village, a handful of hamlets and wonderfully clear water. There is not much to do beyond walking the old paths, swimming and eating well, and that is exactly why people make the effort. Naxos, about 15 nautical miles to the west, is where nearly everyone starts.
The easy way across is the Blue Star ship, which in last year's data sailed on 3 to 5 days a week and took about 1 hr 10 min for as little as €6.49. The other choice is the local Small Cyclades Lines boat, which is also cheap, from €8, but takes almost four hours because it works its way through the other small islands before reaching Donousa. On a calm day that slow loop is a pleasant sightseeing trip; on a windy one it is long.
Because Donousa has no airport and only a few boats a week, the ferry sets the rhythm of your stay. Choose your sailing first and then book your room, and plan the return in the same way, especially if you have a flight to catch from Naxos or a ship to Piraeus.
Donousa also fits neatly into a Small Cyclades loop. From the island you can carry on to Amorgos or Koufonisia, or come back to Naxos and head south to Iraklia and Schinoussa, so it is worth thinking about the whole circuit before booking single legs.