You can go to Shillong,Gangtok or Tawang..I have visited the first two(If you want to see the paradise then go to North Sikkim) You can see my tripoto for picture view..And the single video posted.Video is showing the way to Gurudongmar lake.. You can ask any query related to Sikkim or Meghalaya.
Sikkim is by far the best place to travel solo. It is literally paradise and pretty safe as well. I have been there a few months before and all I got back was beautiful memories. The locals are really helpful and sweet and the place is nothing less than heaven. Have a great solo trip. Cheers
Honestly all of it... each tribe, and there are several within reach state, besides the mainstream state culture, and there are many tribes in each state, had it's own unique culture. And if course each state had it's own tourist hotspots, stop unexploited, that is unique and different. Hence I'd request that please take care of these places and help maintain the still pristine beauty and virginity of the place. From the Eastern Assam to Sikkim to the north to Arunachal in far East- the North East is inexhaustible and an enriching experience. my suggestion would be, provided you have plenty of time, to go unplanned, take it state by state, province by province, v take local means of transportation to experience the more rural areas of these states and go to offbeat small towns of you really want to experience the hospitality and richness of the geography and culture of the peoples of the North East. Although, that can be a bit dangerous given the volatile political situation and the still existing if not really thriving threat of insurgent groups.
In case you don't like the idea then go for the main yours as I see someone below has already provided an excellent itinerary though one could add a few more places but you will know when you travel. thanks