Because of abundant rainfall and warm temperatures throughout the year, both quite normal in Kalimantan, the evolution of nature has taken an interesting path with numerous endemic species present here. Even though some forests of Kalimantan are threatened today, many are still under state protection and are nicely managed in order to attract eco-tourists to help save the existing biodiversity.
That is why a number of forests are well-equipped with trails which allow you to spend from several hours up to several weeks exploring these forests. There are two periods which are the most suitable for such an adventure. From July through September the weather is relatively dry, which promotes mammals and birds to be more active and loud. You'll have a better chance to observe orangutans, Proboscis monkeys, hornbills, and many others. In November and December the rain isn't that hard as well, so you can enjoy lots of fruits and freshly harvested honey.
Pay special attention to Margomulyo Mangrove Forest, which is quite small but allows for a smooth relaxing walk along a wooden trail or a boat ride under the crown of mangrove trees.
Also, Kutai National Park has really well-developed infrastructure. Its area is more than 2,000 square kilometres and you may even find clouded leopards, crocodiles, macaques, and lots of colorful birds.