The weather in Sikkim and Assam is pleasant. In Meghalaya it is raining almost daily if you are considering to stay n Shillong. But if you do visit Shillong, make sure you visit Cherrapunjee as well.
Sikkim seems small. But it's super vast in sense of the amount of places you have to go there. There are so many monastries,so many places of absolute beauty that you can experience there. When I went, I visited Pelling, Yuksom, and Ravongla. All three were amazing.Assam I couldn't explore much. Meghalaya is another magical place. It's like it's plucked and put into this world from fairy tales. While there, be sure to visit Dawki river, Rootbridges of Nongriat, and never ever miss Rainbow Waterfalls. It's a one hour detour from the root bridges. And be prepared to walk a lot ! Thanks !