Mr. Roy's suggestions are excellant - however a little hectic. I suggest a few locations to confuse you.
There is a Forest Deptt. run nature camp at a place called Kalipur near Lataguri town. that is an absolutely off beat location. There are only four cottages, two elephants to ride, a dedicated watch tower called Medhla and a captive area of Garumara NP, where only elephants from Kalipur Forest camp can go in. If luck holds right, you are sure to see lot of rhinos, bisons and elephants. We got saturated by repeated encounters.
At Jaldapara NP, try to stay in Hollong Forest Lodge, run by Forest Deptt. Rhinos and elephants come right under the windows of the three storey bunglow. Seeing wildlife right from the window of your room is a novel experience, again, if luck holds. Take the FIRST elephant ride in the morning and you are almost assured of finding wild life while roaming in the forest. If you stay at these two places, you will see all the wildlife that you hoped to see. From Jaldapara, you can make a day trip to Chilapota forest, if you have a dedicated vehicle. Only thing, you have to hire a local guide, easily available for such trips to Chilapota forest. With the old bunglow burnt down during hill agitations, charm of staying at Samsing is a bit lost. You may go little beyond and stay at the tourist cottages at Suntanekhola, a serene place to get lost in the lap of wonderful nature.
I guess all these, you can book thru net.