Than Sadet-Ko Pha Ngan National Park The park (100Baht, 20 Baht for Thai) is named after the river Than Sadet (literally, "Royal River"). The river forms the largest waterfall on the island, which was visited by several Thai kings. Don't expect much water during dry season. Meanwhile easy to reach on motorbike, even better than some other roads on island. Down at the sea at Than Sadet Beach, there are a few places to stay (fan only) and two restaurants.
Visit the beautiful waterfalls and lookouts in the interior of the island. The best lookouts are Domesila viewpoint, a 15-min hike from Phaeng waterfall in Phaeng National Park (100 THB at the entrance station). There is another waterfall viewpoint a 20-min walk from Phaeng waterfall. From Bottle Beach you can hike to the rocky viewpoint above valley with great views over northeast part of island, hiking to the top should take 30-45 min depending on your condition.
You can visit Wat Khao Tam viewpoint on the road between Thong Sala and Haad Rin which you can reach on motorbike and then it's easy walk for few minutes to the temple with viewpoint at Ko Samui and southern coast of Ko Pha-ngan. Another viewpoint is in Chalok Lam on the road to Haad Khom, it's well signposted from the main road.
Visit Ko Maa off the northwest coast of Ko Pha-ngan. It offers one of the best snorkelling places on the island. Other places to do snorkelling are Haad Khom beach (very shallow water on the coast during low tide, be careful) and Haad Yao.
On the road between Thong Sala and Chalok Lam is a beautiful Chinese Temple (free) overlooking Chalok Lam Bay.
There is an elephant camp on the way from Baan Tai to Thong Nai Pan (300 baht for a 30-min ride. Also one close to The Chinese temple on the road from Thong Sala to Chalok Lam (300 baht for 30-min ride. On the price list it's officially 500 baht). Very close is also an archery range.