Sikkim - Bhutan would be a better option. Reach Sikkim, explore all parts of Sikkim, lachen, Lachung, Nathula Pass and then from Sikkim with the help of tour agencies (based on your budget) you could reach Bhutan via road stay there for 3-5 days explore and return back.
I would suggest for Tawang road trip.There are two routes you can choose according to your comfort. Guwahati - Tezpur - Bhalukpong - Bomdila - Tawang Guwahati - Orang- Shergaon -Rupa - Bomdila - Tawang
Guwahati - Tezpur - Bhalukpong - Bomdilla - Tawang
(Watch World of Mg - Roadtrip across North East India by Tripoto. You will get a lot of knowledge and you'll love it. I have watched it over and over again! Also because of Aisha and Ayush :P :)
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