Some points from my side:
Calcutta - A strict no-no for Mumbaikars. Believe me, u will fall sick.:no: Though the temples, etc are superb (Bellurmath), the area was horrible:no: . Howrah Side is worst, where the above temple is.
But has some good areas too. like Salt lake city, science Centre, etc.
Darjeeling -
All the famous Spots like:
Tiger Hill (Kanchenjunga and Sunrise)
Coupla Monasteries.
Japanese Temple also.
A short Darjeeling Himalayan Rail (Toy Train) Ride to a Station 'X' (around 1 Hr from darjeeling, don't remember da name) & back by local taxi.
Gangtok:
lotsa places to see.
Cable car Ride.
Kanchenjunga view point.
Again few monastries, etc.
Nathula pass. (Indo-Chin Border)
But the best part:
Go to Gangtok (from Darjeeling) via Peling. Most of the crowd goes thru Kalimpong. But, Peling is Superb. Relatively Calm, and unexplored. Small Town situated on the near top of a hill. 'CLOSELY' Surrounded by Green-top mountains on all sides. Very Quiet. Offers the closest view of kanchenjunga. Has the South Asia's highest/Biggest/longest hanging Bridge (something like that...) which is a kinda hair-raising experience. Lotsa BIG waterfalls nearby.
Also, if u have some days left, go to either Gurudongmar (Heaven on Earth, at 17k feet) or proceed to Bhutan. (VISA on the spot. another beauty!!)
PM me for more. I have some Word Docs of that as well.
Regds,
Rutvij.