I did not attempt to travel Northern Sumatra and Aceh by myself as I did not feel comfortable and I am glad I did not. But while we were travelling we met a few singular travellers, some of them young females.
In Indonesia you will get a lot of attention as a bule (white person) but the attention is always friendly in nature. Indonesians are just inquisitive about what you are doing, where you are going, who are you with, are you married, where is your boyfriend, where are you from, do you speak Indonesian, blah blah blah.
One benefit of being a bule is that we never had to stay lost for very long before someone would help us out. A few times we were fortunate that potentially dodgy situations turned out fine.
We were dropped at the wrong port in Parapat, but some local people hailed a bus for us to go to the right port.
We missed our flight to Banda Aceh in Medan and did not want to make the one hour journey back to Medan so were planning to stay the night at the airport (yay, not). However we were approached by a man who offered us a driver to take us to a nearby hotel who would also pick us up in the morning to take our flight to Banda Aceh. A potentially dodgy situation, but everything transpired how the gentleman said, even if the hotel was rancid, but every hotel in Medan is.
Obviously exercise some common sense caution, but I found Indonesia – Sumatra to be as safe if not safer than metropolitan Melbourne.
I definitely would not do the jungle hike alone and would not split off from the person I was travelling with in the jungle. Sometimes guides can be overzealous in divulging their sexual experiences when they have young females enclosed in the jungle. But is a product of some common Indonesian (if not many more countries) views of western women.
Just use your common sense