That sounds like a wonderful trip. My paternal grandfather and his ancestor were from Chaksi[Disallowed String for - castist references disallowed] while is pretty close to Taplejung and my mom and her whole family is from Darjeeling. Last time I was in Taplejung was in 1995. I bet it has changed a lot since then. If you plan on going to Darjeeling be sure to visit Chowrasta and Tiger Hill.  Kurseong and Mirik are just breathtaking from the train to tea estates. Sonada is a small town few miles from Darjeeling bazaar but it's beautiful. Make sure to have some authentic Darjeeling Momo and Thukpa.

 If you go to Sikkim one of the the best place to visit is Rumtek Gumba (Monastery) in Gangtok. Kyongnosla waterfall on the Nathula Pass on Gangtok highway is amazing. Best way to see Gangtok is hire a private taxi and drive around the town. Their local town bazzar is amazing, lots of local goodies and Churpi on string is the best. Their pastries/bakery is amazing. I went there when I was 6 years old and stayed there for about a month but I can still remember all the fun I had and all the besutiful places I visited during my stay there.

Have a wonderful journey and looking forward to all your pictures