It isn’t easy to travel by road but flights are cheap. On the other hand if you go by minivan you will see so much more Sumatra...
Sumatra is suprising - mountains, pine forests, mild climate suprisingly for equator it could often get chilly. Lots of agriculture. Vivid nature. But real jungle and primary forrest is mostly gone.
People are friendly, smiling, easygoing and welcomming to foreigners. Lots of people speak a little English and bahasa is quite easy to pick up. No grammar, super simple.
Indonesia has almost worst education system in the world [1] and forward thinking isn’t part of the culture. Neither is working hard. That can put you through lots of hardship - if you care.
People are incredibly social - this is what their life is about. There is a lot of childish fun everywhere - when minivan is passing through the village local people pour buckets of water through open windows. Lots of people sing and play guitar - most badly in my opinion but it is still sweet.
People are honest, you can trust strangers, not much of small crime, petty theft is uncommon etc.
Everyone smokes cigarettes everywhere also on the bus. Even 4 year old could be already hooked on fags. [2]
There is a lot of people, it is quite overpopulated island (not like Bali but close). It means tons of trash everywhere, everywhere, terrible impact on environment. Indonesia is world largest producer of marine pollution (3.2 milion metric tones of plastic per year). [3]
If there is trash it is mostly burned on site (including plastic et all) so you get a haze. Cities are polutted, there is too many cars, people and not enough infrastructure. Medan - largest city in Sumatra and 2nd largest in Indonesia is the worst place in SE Asia in my opinion. It is just hudge, poor, dirty and without anything there to compensate.
Clean water, sewage’s treatment forget about it. Everything goes into rivers and ocean. There is now wild gold rush which is destroying riverbeds in Sumatra.[5]
The food is good but monotonous.[4] The hotels, homestays, peningapans are usually quite dirty. Regarding cleanliness Thailand it is not. Don’t expect clean sheets. Bedbugs are common. There are exceptions.
Definitely go to Sumatra.
Disclosure: I have spent mist part of last four years here.
[1] https://www.oecd.org/pisa/PISA-2015-Indonesia.pdf
[2] http://edition.cnn.com/2017/08/30/health/chain-smoking-child...
[3] https://ehp.niehs.nih.gov/wp-content/uploads/123/4/ehp.123-a...
[4] There are miracles like organic yogurt, mozarella and mascarpone from Medan or italian restaurant Bixio in Pulau Weh.
[5] https://thediplomat.com/2017/10/making-indonesian-rivers-gre...