What to do:
The area seems reasonable for water sports, and some of the reefs offer decent snorkeling, although many of them have been dynamited in the past and so most coral is still just recovering in patches. It also looks like a number of the beaches included Pantai Kalampunian could hold a decent swell, so surfing could be in the cards, and it seemed like boards could be rented from the backpackers. Definitely go check out the confluence of the two seas at the Tip of Borneo and beyond that, I highly recommend a little beach hunting.
Pack a lunch, some water, and a camera, and wait for the tide to start going out. When it is low enough to cross onto the rocks at the southern end of Pantai Kalampunian Beach, cautiously traverse around the rocky point being super careful with the deceptively dangerous slippery rocks, and keep an eye out for the amphibious walking fish (mudskippers) that hop around the surf line. As the tide ebbs, tide pools will form and race car red crabs will scurry in and out of crevices of periwinkle barnacles.
The rocky point will lead you to a beautiful crescent moon beach, back dropped by a dense mixture of coconut and pine tree foliage, mixed with tall sweeping pandanus grass, and jungle vines. This walk will take you through a series of secluded little white sand, crescent moon beaches, but only after the first three (at Pantai Avoi Beach) will they be deep enough to swim in. Here, you can climb up to the look-out for a picnic, or just go down and lay the day away on the beach. This is the beach in front of the Hibiscus Villa, so alternatively, if you are staying there, you could do this walk from there to the Tip of Borneo.
You will know the tide changes and starts coming in as all the brittle stars turn over. Instead of rushing back over perilously slippery rocks, you can walk up the dirt road from Pantai Avoi Beach on to the main road and saunter back to wherever you have chosen to rest your head for the night.