On a day tour from Phuket, Patong beach I took a tour to Ko Similan. It wasn't cheap then, so it will probably won't be cheap now. There was an option to stay overnight, but it was camping, which I don't like.
We drove to a boat dock where we were left the main land to the islands. It was in big speedboat, make sure you'll take motion sickness pills!
I can't remember which islands we visited, but they all looked lovely with white sand beaches and crystal clear water.
At a few you can hike up a hill to see a viewpoint.