What are some non-beachy activities/sights in Koh Samui?

There are a few waterfalls that you can hike up and swim in. There are many temples(Wats) to go to. If you can ride a scooter, rent one. I made it my mission to go down every single road on the island and did most of them twice. I came across many things that no tourists ever see. There are deserted beaches and local villages and markets if you go as far as possible away from Cheweng and the other main tourist beaches. It's an amazing island to get lost and explore. If you can't ride a scooter there are cars for hire for a bit more money. My advice is get lost as much as you can, it's easy to figure a way home with a basic map and road signs or a smartphone.

Sara, all the beaches in Lamai Beach and Chaweng is the most popular.