North East is vast with many tourism circuits, you may not be able to see everything in one trip, so decide on a specific place - Sikkim / Meghalaya / Arunachal Pradesh / Nagaland or Bhutan. All places are equally beautiful but if travelling to that region first time, plan for a 7 days trip to Sikkim covering West and North Sikkim besides Gangtok etc.
Https://www.tripoto.com/india/trips/north-east-pride-of-india-9944
Kindly check the above Tripoto link for more details. I will share some offbeat north east spots see if it is suitable for your travel. Namphake is a mesmerising village
Molung popular for Changkikong ranges
Andro foothills of Manipur
Kongthong called as jingrwai lawbei
Bheloghat falls at anglong district
Tuophema Nagaland village
Solomon temple aizawl Shergaon west Kameng district.
Guwahati and Shillong are great! There is no special pass to go to these two (unlike some other states there which require registration ahead of time). The Elephant Falls in Shillong is lovely, and the countryside around Guwahati was of course breathtaking
7 sisters are hard to reach and explore so before that comes the only brother of 7 sisters called sikkim. it's the best place to start with if you have never been to North east. I would mostly request west towards pelling as its the best place to be. there is no restriction there or any permit required and one can enjoy with friends and see a lot of things around