Hey Sekhar, In this amount you can cover many places. I would suggest you to go for this route : Siliguri->Darjeeling (2 days)->Gangtok(2 days)->North-Sikkim(3 Days)->Gangtok->Guwahati(2Days)->Shillong( 3 Days- covering Cheerrapunji,Dawki). I covered this all in a single tour in 16 Days and total cost per person was Rs 20K ( on my own itinerary as it becomes much cheaper and more flexible than booking a package).
In this budget you can also travel to laddakh. If you want to go to an off beat destination, go to jibhi jalori. Kadol kheergang tosh is also a good option. You can also go to Darjeeling or Sikkim