Itineray advice/suggestions East-India

zaaaaaaara

I will be leaving for India in the middle of the next month for 4 weeks and would like some advice about my trip route...
Day 1-3: Delhi
Day 4: Leave for Allahabad
Day 5-6: Allahabad
Day 7: Leave for Varanasi
Day 8: Varanasi
Day 9: Leave for Calcutta
Day 10-13: Calcutta
Day 13: Overnight train to Jalpaiguri
Day 14: Jalpaiguri
Day 14-16: Darjeeling
Day 17: Hire taxi in morning to get to Jalpaiguri for trip back to Calcutta
Day 18: Catch Train to Bhubaneshwar
Day 19-20: Bhubaneshwar sightseeing
Day 20: Evening Hire taxi to drive to Puri (2 hours away)
Days 21-22: Puri
Day 22: Catch train at Bhubaneshwar to Cal.
Day 23: Half day sightseeing at Calcutta and catch train to Delhi
Day 24: Delhi
Day 25: Catch train to Agra
Day 26: Stay at Agra and hire taxi to Mathura
Day 27: From Vrindavan take taxi to Bharatpur National Park
Day 27: Back to Agra, Catch train/Bus to Delhi
Day 27-30: Delhi

What do you think? Anything on the way I should not miss? Delhi, Agra, Varanasi and Kolkata are MUST, but the other cities - changable. Looking forward to your opinions