Sumatra..the first question really is...are you male or female?
Travelling Sumatra as a female solo particularly in Aceh where you would land as a jump off point to Weh...and Aceh is under Shariah law so as a solo female it was a city where I had to give a bit of extra consideration to my dress and behaviour. Out on Weh itself Lumpa Lumpa divers
Diving in Sumatra
I must say after watching the clip above..you should've seen it 6 or 7 years ago.
Hoping someone with more recent experience in Weh can chime in - I do know it has become very very popular with new divers from Singapore and is a reflection of the emerging Asian diver market.
I like an island off the west coast in the Indian Ocean reached from Padang called Cubadak for chilling and diving. Being the Indian Ocean the diving is different but last I was there surprisingly interesting. There were a lot more bigger groupers etc than other areas - mixed bag, you will get nudi's and a lot of Cephalopoda and sponge gardens, but not your Blue Magic or Mayhem schools of fish. It's part of the mentawai's which are best known as surf destinations but really interesting culturally. Local tribes on the islands are still mostly living as they always have been. The Ambu Ambu Ferry from Padang will get you there. There is also the Riau Archipeligo dive sites in the link posted.
Lake Toba, Bukkittingi to meet the Mingankabau people with their amazing architecture and traditions and yes, the Orangutan centre is great.
If you do make it down to Chochoroc port where you leave for Cubadak the road from Padang may yield an amazing surprise if you are lucky. I spotted Pygmy elephants drinking from the river and two orangutangs in the wild. There are also amazing "living bridges" where tree roots have been trained for hundreds of years to create bridges.
Sumatra overall is not the easiest of islands given road systems, distance (it's big!), religious issues can be a hassle and if they are burning to clear land forget Sumatra because the smoke haze blankets Singapore sadly a yearly occurrence.
Lots of Natural Hazards but....I love Sumatra and do think it's worth the little extra effort you have to put in to see this part of Indo.