The roughly circular island of Donousa (pop. 160) is about 15km east of Naxos. It's only 4km in diameter, and is part of the half dozen or so "Small Cyclades." Donousa is set apart from the other Small Cyclades, which lie in a tight group some 30km to the south and east, of off the south coast of Naxos.
Donousa's main charms are its seclusion, its lack of development, and its very nice, very quiet beaches. This is the place to go to get a breather from the much larger and more crowded Naxos, which has regular ferry service to and from the islet.
Accomodations are a little scarce, and are found in the port town, also named Donousa. On the other hand, Donousa is close enough to Naxos to make for a decent day trip. The beaches are not organized, but they are beautiful, and chances are that you can have one all to yourself, depending on the time of year.
The largest beach, Kendros, is 3km east of the port. It's the largest and "busiest" ("busy" being a strictly relative term here. Compared to some beaches on Naxos, it's practically deserted) on Donousa, and a favorite for primitive campers. It can also attract nudists, so if you have small kids you might want to steer clear of it. It's fairly well sheltered from high winds. Access is provided by hired boat. An added feature is the sunken wreck of a German boat dating from WWII. It can clearly be seen on the sea bed.
The Port Beach is the closest to town. It's the only beach that attracts crowds because it's so close to Donousa Town. Again, the word "crowds" doesn't quite do this quiet place justice. Five hundred meters from the port, it is a long, beautifully sandy stretch of waterfront. It's perfectly fine, with crystal clear water, for day trippers, with the added advantage of being close to the restaurants of the port, of which there are only 2, as well as a small supermarket. There are a few other beaches scattered around the coast of Donousa, all of which are reachable by boat for hire from the port, by foot, or by bike or car.
You can cover most of the islet by foot in just a few hours, which makes for very pleasant walking, if you wish to avoid people, because you'll rarely see any on such a walk. The sea is always close at hand if you want to cool off, and chances are it will be just you and your companion to enjoy it. Donousa is a good place to have a phrase book handy, since most of its inhabitants know little English.