Need Help With Kolkata-Siliguri-Darjeeling-Kalimpong-Gangtok Budget Trip in Jan 2012

Ill post the tentative first. My queries are mentioned below the itinerary.

Bengal Trip Itinerary (29th Dec 2011- 20th Jan 2012)

29th Dec: Train at 6 am from CST (Mumbai)- Gitanjali Express

30th Dec: Reach Howrah Station by noon. Get lodged, and leave for sightseeing.

31st Dec: Sightseeing. Celebrate New Year at appropriate place.

1st Jan: Sightseeing.

2nd Jan: Sightseeing.

3rd Jan: Leave for Sunderbans early in the morning.

4th Jan: Sunderbans tour.

5th Jan: Leave back for Kolkata. Reach Kolkata by evening.

6th Jan: Leave for Shantiniketan (By Ganadevta Exp at 6 am). Back to Kolkata by evening.

7th Jan: Leave for Siliguri by train at night by 8:30 pm train - KanchanKanya Express.

8th Jan: Reach Siliguri at 8 am. Rest of the day - sightseeing.

9th Jan: Leave early morning for Darjeeling. Spend day in sightseeing.

10th Jan: Darjeeling sightseeing

11th Jan: Leave for Kalimpong in the morning. Rest of the day sightseeing.

12th Jan: Kalimpong sightseeing by day. Leave for Gangtok at noon.

13th Jan: Gangtok sightseeing.

14th Jan: Gangtok sightseeing.

15th Jan: Gangtok sightseeing.

16th Jan: Leave back for Siliguri in the morning. Train to Kolkata at 8 pm.

17th Jan: Reach Kolkata around 8:15 am. Sightseeing.

18th Jan: Rest/sightseeing/shopping.

19th Jan: Leave for Mumbai by Gitanjali Express at 1:50 pm.

20th Jan: Reach Mumbai by 9 pm.

Total of 23 days.

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The days - 29,6 entire day, 7th night, 16 night are fixed up as we would be traveling by train and tickets have been booked.

Rest of the days we are free to do as we like, depending on the situation and place will decide what and how to go about touring.

Places we wish to see in Kolkata include:
Science city, Indian Museum, Victoria Temple, Dakshineshwar Temple, Nico Park, Birla Planetarium, Belur Math.

Other than that, dont have much of an Idea about where to go.

At Gantok, we have 3 full days, so thinking of visiting Pelling one day, Yumthang the next day and Nathula Pass on the 3rd.

Could you friends help me out with the places worth visiting and if possible in what order(especially in Kolkata)and how to go about them.

We are a group of students(total 26 in nos) consisting of girls and boys. We are trying to save as much as possible through out the trip. It would help if one could give us ideas about how to save money as in with cheap lodging options,food, travel etc.

I also wanted to know would it be safe to have 12 girls traveling with us? I have heard about safety concerns at places and so am worried.

What would be the best procedure for me to tour Sunderbans with the group keeping in mind that we want to visit with minimum possible money being spent.

We prefer a self managed touring rather than through tour agencies or things like that. That way it is much more enjoyable. Other than the days where tickets for trains are cofirmed, we are open to any changes that may be required on the trip

Kindly take a look at the itinerary and suggest improvements or changes if required

P.S. last year we made a trip to Rajasthan(Chittaurgarh-Udaipur-Ajmer-Jaipur-Bikaner-Jaisalmer-Jodhpur-Mount Abu) trip in 12 days that costed us INR 4500 per person only (8 of us) .
This is a much longer trip and the amount also increases considerably. On a similar line, we are trying to replicate the low cost trip.

Thank you in advance