The boattrip over the river into the rainforest of tanjung national park was wonderful! The guide, Ancis, knew a lot about the animals and vegetation in the different landscapes and was very easy to talk to. He used to a ranger. We saw many orang oetangs, other monkeys and even a crocodile!
If you are planning to make such a trip be sure that you do not leave traces, take care of trash (including toilet paper) and try not to interupt the pure nature. As tourist we are just visitors.