You can enjoy an in-depth tour of North-east in 8 days. Here is a tentative itinerary that you can enjoy: Day 1: On day 1, you can reach Gangtok and stroll across the market of Gangtok.
Day 2: You can embark on a sightseeing tour of Gangtok and explore destinations such as Namgyal Institute of Tibetology and Do Drul Chorten.
Day 3: You can head to Tsomgo Lake
Day 4: You can head to a tour of Kalimpong and visit places such as it Zang Dog Palri Fo Brang Monastery and Tharpa Choling Monastery.
Day 5: On Day 5, you can head to Darjeeling.
Day 6: You can enjoy a one-day trek to Tumling. Day 7: You can explore tourist destinations such as Himalayan Mountaineering Institute, Zoological Park and Tenzing Rock.
Day 8: On Day 8, you can explore the market and prepare for departure.