I originally went to Sumatra for 2 weeks but ended up staying 5, this island really pulls you in. It’s not the easiest place to travel: expect delays, long bus rides, and rough roads, but the rewards are incredible. I felt safe everywhere, met welcoming locals, and loved that in many spots I was the only foreign traveler. Cash is essential (cards are basically useless), buses and shared taxis are cheap, and hitchhiking was surprisingly easy and fun.
Highlights were the jungle treks, camping at 3,800m on Kerinci, and snorkeling on Pulau Weh. If you go, give yourself plenty of time. Sumatra isn’t about rushing.