Hi,
You could explore on your own, or if you search the web, there's private nature guide available for hire.
At the park's main gate, there are boat operators that can bring you on boat trip to Pantai Kerachut/Meromictic lake/Turtle sanctuary/Mangrove, the duration is about 4hrs, the charges is based on per boat, which can take 10-12 people.
In July, we(group of 8 people, including 5 elders>55yo) trekked to Monkey Beach, and return by boat. Took us some 3hrs to trek there, but it was only 5mins boat ride back.
By the way, there's a canopy walk in the park, the tickets are sold at the information and registration office at the park entrance only, remember to get the tickets first before trek all the way to the canopy walk.
Have seen bus101 and 102 stopped at the gate.
Take lots of water and some snacks too, no food/drinks seller in the park