I did in fact end up going down to the southern tip of Ko Lanta to visit the National Park. Calling it a "national park" is an overstatement, as it is more of just a "park." Technically all of Ko Lanta is a national park, although people live all over so it is unclear exactly what the designation means.
At the Official Ko Lanta National Park, there is a lighthouse, a rocky beach, a sandy beach, a park area with a dirt soccer field, and a 2 km walking trail. I went up to say hi to the lighthouse and walked around the 100% empty sandy beach.
After sitting in the shade for a bit to let my body stop profusely sweating, I wandered around the park for a bit. Then, to my pleasant surprise, some monkeys came to play. Well, they really came to eat as there were some Thai dudes with food. Despite all the signs saying DON'T FEED THE MONKEYS, it is definitely everyone's very first thing they do when monkeys show up.
I think monkeys are amazing, and before Thailand had only seen them in captivity- never just chilling in the wild. After getting whatever human food they could get, the monkeys wandered around the beach to look for more food. They caught and ate some mini sand crabs and cracked open some random nuts. I followed them around for quite awhile, since watching them was amazing. Eventually they wandered away from the beach and back to wherever their monkey house is. Before they got too far away, I turned on a shower hoping that a monkey would drink some water. My monkey whispering must have been strong that day, as one of the little dudes happily obliged. He scampered up to the top of the shower and enjoyed some fresh water. My day was made. The rest of the photos from the day are below. Click any image to start a larger slide show.