Meghalaya is best option for October.
Places to visit
1) Shillong- capital, shopping.
2) Cherapunji region- living root bridge, Rainbowfalls, Nohkitalfall, caves.
3) Mawlynnong- Cleanest village in Asia.
4) Dawki- crystal clear water, boating, river Crossing.
5) David Scott trail.
6) Bamboo trek.
If you have 4nights you can plan for North Sikkim
1st Day- Bagdogra/ NJP to Gangtok. and halfday Sightseeing at Gangtok. At evening make your permit for North Sikkim. You need to submit your photo and one copy of your ID proof to any registered tour operator of Sikkim. 2nd Day - Gangtok to Lachen. Nightstay at Lachen. 3rd Day- Lachen to Gurudongmar to Lachung and Nightstay at Lachung. 4th Day- Lachung to Yumthang Valley and Zero Point and back to Gangtok