Month in North India - from Delhi to Darjeeling and back

Angelos

Hi,

me and a friend of mine are planning a trip to the North of India. We are arriving to Delhi on Feburary 21st and leaving on March 21st. That is 29 days. A must see for us is Darjeeling, where, if possible, we would like to do 4-days of trekking and then we would move to the Sikkim. However we are not sure about what we want to see in Uttar Pradesh and Bihar. I would like to see some National Park, but I don't know if there is any good in this area. So maybe we could just go quickly to the east and go to Assam for Kaziranga and Guwahati. In Uttar Pradesh there are also lots of places to see. We would definitely like to see Varanasi and (again if possible) Khajuraho, but there are still places like Fatehpur Sikir, Gwalior, Orcha, Bodhgaya, Calcutta etc.
Can you help us to do some itinerary? We don't really want to hurry there, so rather less places than more. We are going to travel on a lower budget. We would like to spend on average not more than 15$ per day.
Thanks a lot for your advice!