My boyfriend and I went to Thassos a couple of years ago and stayed in Thassos Town. We loved it and would definitely go again. The ferry trip across to the island put off families with small children so it was mainly couples and families with older children.
The ferry journey across was hassle free as you sit and have a drink in a taverna by the sea and wait until the coach has boarded before you get on the ferry. The journey across was lovely and you could see the island in the distance looming closer.
There are plenty of bars in the town to choose from and lots of restaurants by the sea, in the main street or up the smaller side streets. You can make it as low key as you like depending on which area you go to. Lively main street bars or small local bar on the outskirts.
We loved sitting in the tavernas on the town beach with our feet in the sand watching the ferries going back and forth to Kavala.
The sea is crystal clear and the boat trip was superb!
Failing that, how about Kefalonia? Beautiful island, wonderful dramatic scenery, nice beaches and low key nightlife.