I visited India many years ago, travelling around this country for two months. I took a few photographs which I have since used in lectures, although they are crusty old pre-digital 'film' shots. Uck. As usual, I tried to survey a range of different habitats.
Travelling around India is both a pleasure and a challenge. First of all, it is very cheap; to get around, to eat, and accommodation is generally very good value. It is a big country, with a huge population, and the accompanying congestion, so travel takes time. It is a pretty 'full on' experience, with crowds, and if you are a single westerner, you will be the object of attention. In the middle of my 'holiday' I had to take another 'holiday' in Nepal to have a rest from India.
There are several areas worth visiting from a natural history point of view. India is in some ways the 'Africa of Asia', with less rainforest than south-east Asia, more open woodlands and grasslands, and thus larger animals. This is the best country in Asia to see megafauna such as elephant, deer and tiger.
First, my favourite spot was Periyar National Park, in the south. This is a valley that has bee flooded into a dam. It is surrounded by tropical rainforest. There was a hide available to stay in overnight, and here I was lucky to watch elephants, gaur and sunbirds. In central India is Kanha National Park. You can drive around this park to try and find tigers (which I did), there are also elephants, deer, dhole, monkey, etc. In the north, just outside New Delhi is the Bharatpur Bird Sanctuary. Even though this small wetland is surrounded by humans, and has a huge amount of people visiting, it is (or was when I was there), chock full of birds, including many ducks and kingfishers, as well as reptiles such as pythons.