We stayed 6 nights at the Renaissance Koh Samui for our honeymoon and we highly recommend it! Our villa was very comfortable, the amenities were great, and the environment was very beautiful.
We also enjoyed the beachside pool, the massages we got at the spa, and all the food we ate at their restaurants.
We booked a number of experiences outside of the hotel (not through the hotel), specifically a cruise around Mu Ko Ang Thong National Park, a Thai cooking class, and a visit to an elephant sanctuary, and all of them were able to pick us up directly from the hotel.