I stayed at Merlyn's guesthouse and would recommend it for sure. The very private water access is the biggest selling point. Although it seemed fun to stay in the traditional Batak style accommodation at Merlyn's I chose a regular room for a little bit more.
Two highlights on the island are the ancient stone chairs were judgements and executions were decided upon by the Batak (the local ancestry) elders in Ambarita. As well, the Batak museum in Simanindo has some neat Batak style houses and Batak artifacts on display. Be attentive while heading to these places because the roads on the island are not good and the attractions are easily missed due a lack of signage.
Garden Inn, Bukit Lawang
I will provide this warning: When you arrive in town you will be swarmed by people asking to take you somewhere to stay or to see the orang-utans. Be polite and say no thank you. You will most definitely see these people again and will maybe stay at their guesthouse in the end.
Bukit Lawang is a long narrow village along a river. It gets very hot in the rainforest climate and walking can get tiring. Opting for cheap accommodation over immediate comfort I ended up walking for over an hour with my pack in the middle of the day to find the Garden Inn. The narrow, root disturbed, pathways leading through town can mislead you into thinking you are walking straight out of it, but don't panic, you want to be staying on the outskirts.
The Garden Inn was a lovely place to stay. The staff was extremely friendly and helpful. The rooms were amazing and scenic and from the breakfast table I could see Orang-utans swinging in the trees across the river. Not to mention, the Inn is close to some fairly private river access for undisturbed swimming.
I saw them on two separate occasions. The first was a day long hike that took us into some pretty difficult jungle terrain, but paid off huge as we saw multiple orang-utans, ate lunch with some monkeys, and then were approached by a large Mena Orang-utan and her baby. Mother and baby were a once in a lifetime sight for sure.
The other occasion was at a feeding centre just minutes from my guesthouse. Park rangers brought bananas and orang-utans appeared out of the jungle from all sides.