You can't cover the entire North East India as a single trip... You can cover Darjeeling-Sikkim as one trip in which you can cover: Darjeeling
Gangtok
Gurudongmar Lake
Tsongmo Lake
Yumthang Valley
Zero Point
Nathula Pass
Baba Mandir
Pelling
Ravangla You should cover Assam-Meghalaya-Arunacahal as one trip... Guwahati
Kaziranga National Park
Majuli Island
Shillong
Dawki
Mawlynnong
Cherrapunjee
Nohkalikai Waterfalls
Tawang