When traveling in Borneo keep in mind that distances between two destinations are usually huge and that most activities are quite expensive. You can travel Borneo on a budget but you'll be missing on a lot of great adventures.
Best things to do in Kalimantan:
1- Travel up the Mahakam River
Going up the Mahakam river is truly a unique experience, the farther you get from the coast the deeper you enter Kalimantan wild rainforest. Traveling along the Mahakam river up to the heart of Borneo is the best way to observe the lush jungle and the Dayak culture. It's also the place where you'll see the last few remaining Irrawaddy pink dolphins, almost extinct now.
If you are in the area in August, you might want to get tickets to the Erau International Folk and Art Festival. Tribes from all over the world perform during this festival.
2 - Surpass yourself with the Cross Borneo Trek
This is the ultimate jungle trekking experience, you'll need time, money and strength to achieve this 7 days trek across the Bornean rain forest. It's a challenge, you'll need to be physically prepared for this. You'll also need a good budget since you'll need to hire a guide and go through an agency.
After going up the Mahakam river, you'll start the trek at Tiong Ohang, trek across the Muller Mountains and take the boat down the Kapuas River. The Kapuas Hulu area is great to immerse in Dayak culture and visit some of the few remaining longhouses in Kalimantan.
A 15 days trek cost about 4000 USD per person (including the transportation by boat from the east cost to the starting point and to the west coast from the finish point).
3 - Dive in the Derawan archipelago
Derawan archipelago is where you switch your hiking shoes for some well deserved relaxation time. From the 31 islands you'll find there, 4 are equipped to welcome tourists. Derawan is the most accessible and also the most crowed, mostly by local tourists coming over the week-end. Maratua atoll is more of a peaceful paradise and Sangalaki and Kakaban are wilder, with dense forests.
The Derawan archipelago is one of the best spot in Indonesia for diving and snorkeling. The reefs are incredibly well preserved and the marine life really diverse. Among the most commons marine wildlife, you might see barracudas, sharks, mantas, and turtles. The archipelago is also famous for its jellyfish lake in which you can swim.
Getting to Derawan is easy, there's a regular boat service from Tanjung Batu for 100k rp. To other islands you'll need to charter a boat and the price can be quite expensive. Same, once you're in Derawan, the prices of island hopping is pretty expensive. Some hotels offer packages with lower prices, but you'll still need to have a comfy budget if you want to fully experience the archipelago.
4 - Try bamboo rafting in Loksado
Nestled in the mountains in southern Borneo, Loksado is the best and most accessible place for trekking. It's also a good place to observe Bukit Dayak's culture and traditions, and of course do some bamboo rafting down the river. While trekking expect to see numerous waterfalls and several remote Dayak villages.
5 - Explore the floating market in Banjarmasin
Banjarmasin is home to several important floating markets, another unique experience, especially as floating markets are becoming more and more rare. You'll need to leave early morning and arrange for a boat in advance. It's surprisingly untouristy and the sellers are not all over you. It's also a goop opportunity to try some local fruits.
6 - Meet with semi-wild Orang Outans in Tanjung Puting National Park
This National Park is one of the most famous attraction in Kalimantan. There, you're guaranteed to see semi-wild Orang Outans. They rehabilitate the babies and after, free them in the jungle. They have feeding time where the Orang Outans are free to come if they want to or not.
The park is only accessible by boat, the boat ride is epic with many chances to spot wildlife. It's also a good place for trekking, unfortunately it's mandatory to hire a guide. It's recommend to spend at least two nights at the park if you don't want to rush.
Beware that the price is relatively high, the cheaper way if you're not a large group is to go through an agency. If you're planning on going to Sumatra or to the Malaysian part of Borneo you'll find way cheaper options to see Orang Outans. Count at least 200 USD for a 3 days 2 nights trip.
7 - Go off-the-beaten-path in Danau Sentarum National Park
Danau Sentarum National Park is one of the best places to see wildlife in Kalimantan. The park mainly constituted of swamp, lowland forest and seasonal grassland is home to 237 bird and 143 mammal species.
There are two entrance points: Lanjak in the north and Simitau in the south, you'll need to register at one of them (150k/day). You'll also need to arrange for a guide (150K/day) and for a boat (400k to 700k/day). The park is inhabited and counts about 20 villages, it's a good opportunity to do a homestay in one of the longhouses there.
You can organise many local activities such as fishing, honey "hunting" and so on.
8 - Join the Cap Goh Meh festival in Singkawang
This busy Chinese town holds a colourful ceremony each year during the 15th day of the lunar new year. Famous for its huge night market, classic shop houses and about 1000 temples, the city lighten up for the ceremony where Chinese dragons and lions mix with Dayak holy men.