The Gunung Lauser National Park is special as it is one of the few places on earth where you can see the Orang Utan (ind. for Man of the Forest) live wild. However due to the rehabilitation program the Orang Utans seen very adapted to humans and because tourist groups leave their fruit snacks along the hiking paths there is a natural attraction for the apes to come close. We saw eleven in total incl. male with typical face and many mother/ child duos. On day 2 and 3 of our hike we were also alone with two orang utans each which was amazing. Of course it's not all about Orang Utan in this tropical rainforest, but it surely is the highlight. We also saw black gibbons, white gibbons, long tail makakes, Thomas Leaf monkeys (endemic to Sumatra) and pigtail makakes. We didn't see any birds despite watching for them. We heard a few though. You can also walk in 7 or 8 days through the forest from Bukit Lawang until Kutacane and go back by Bus in about 15 hours via Medan or in about 20 days until Gunung Lauser mountain and raft back to Bukit Lawang. With at least 7 days there is the option to see wild elephants. Also our guide has seen a tiger once in his career of 7 years and about 7 times a python. He had never seen rhinos or nocturnal leopards.  
 
As all the other tourists we went to Bukit Lawang however with nothing pre-arranged. The local bus from Medan costs 25.000 IDR for locals and according to the driver 50.000 for tourists (I think tourists are people with white skin in this case). There is no luggage storage and both road and bus are not in good condition. It starts at  Pinang Baris Terminal in Medan but we entered from the closeby Mawar bakery and cake shop to have less touts annoying us. From the final stop it is a five minute Tuk Tuk ride to the Guide association were you can book your jungle treck. Also all the hotels offer the service to book it for you and it seems to be the same price. Our guide picked us up when we left the bus but we booked him as we had a good feeling and it proofed right. We paid 170 EUR per person for 3 days/ 2 nights. 
 
The day after we started the hike with only the two of us and Anuar L. as guide from our hotel through a rubber tree plantage into the jungle. Already next to On the Rocks bungalow we saw the first Orang Utan mum and kid in the tree. However with us there were twenty other tourists. And it's low season now. High season is much more busy said Anuar. It's in June to September when it is more dry. The hiking is really tough as you walk steeply up and down the hilly forest and every day in the afternoon a heavy rainfall commenced which caused the paths to become very slippery affording walking on hands partly as well. The camps are bamboo roofs on bamboo sticks and protect you from the rain. However your clothes will not dry from rain and sweat until the next day so it's better to carry many dry clothes and water protection for your bag as well. Under the roof a mosquito net is provided and mattresses and some hard pillows. We had one camp construction for us which was good to dry at least a bit the clothes but in high season it's really crowded with low privacy level in the camp. The longtail mskakes come in the morning to pick up the rest of the breakfast and seem to be a bit aggressive and forcing. At night you have many insects. I got despite protection many mosquito bites and the lowlight was a  caterpillar crawling in my neck in the camp causing strong skin irritations that lasted days. Also the bleeches are disgusting as the wounds don't stop bleeding but at least they cause no pain. We had 1 blech each is 3 days. The food provided by the 17 year old cook was awesome and a lot. It seems not healthy though what the cooks and assistants carry through the jungle in terms of weight though. The last day we rafted with all our luggage back on Bokorok river to Bukit Lawang which was fun. The huge raftings rings are also carried all the way to the camp by the cook.  
 
On another day we made a motorbike tour to the tallest flower in the world. It was still blooming but already starting to die. Also we saw another older version of the flower which already turned into fruits as our guide explained us. In total it was a 3 hour excursion from Bukit Lawang. 
 
Of course there are also two other national parks included in the WHS that I haven't seen and I don't know how they can be seen. However I will already rate the WHS based on Gunung Lauser NP. It'a a typical tropical rainforest with the big specialty of Orang Utans and some other interesting monkeys. Without the Orang Utan I would not rate the park much more than a medium rating however the Orang Utan really makes it to a special place. 
 
In the end of our trip we met a British scientist studying Orang Utans. She doesn't like Bukit Lawang because she gets angry with how all the guides feed the animals by leaving fruits behind. So if you want to do something good for the NP and WHS you can collect the rubbish of others.