I had a beautiful visit in Donoussa staying at Ιωαννα's place. The apartment location is great, it's a less than 5 minute walk to the main Stavros beach, near several restaurants, markets, and bakeries and very close to the port. Avli was the closest restaurant and my favorite. It is a little more expensive than the others but worth it.
About the apartment: it's very minimal but it is clean and the AC works. There's lots of room to spread out/store items and having a water facing patio was great to watch sunrise/sunset. My only real complaint would be that the WIFI didn't work in the bedroom, but not a huge deal since I wasn't using it much.
Ιωαννα let me use another vacant, nearby apartment on my last stay to store my suitcase which was SO appreciated as my checkout was at 11AM and the ferry did not arrive until 10PM.
A couple recommendations for people visiting: the other beaches are 100% worth going to. Kedros is about a 25 minute walk away with a nice beach bar at it. Livadi was my absolute favorite beach and is reachable by the bus or small boat that travels around the island. Kalotaritissa is reachable by bus and has 3 smaller beaches all close together (these are a bit more rockier, less sand, but still beautiful). The boat that ferries people around the island also stops at Fokospilia which is a beautiful swimming cave. The apartment location makes it easy to use get to the boat/bus port to get to these places.
I would 100% recommend either bringing a beach umbrella or buying one on the island (though the ones on the island are pretty cheap quality) as there is very little shade on the beaches.
I had a lovely stay at Donoussa, would definitely recommend visiting. Thank you Ιωαννα for helping make my stay a good one!