Welcome back! I see that your much awaited trip to Bhutan is nearly at hand. And after main course of this little country which is packed with such an immense number of things to marvel at, it seems only fit to sign off the trip with some pretty delectable desert from our own country. Most happy to oblige.

A Limbu Lady in front of a temple (Source: Sukanto Debnath) 

I agree that travel time becomes a matter or concern, especially after a trip to the untamed and rugged Bhutan. Keeping that in mind, the places that you have mentioned become quite strenuous.

Kaziranga National Park is a 9 hours drive from Jaigaon. As for the Sunderbans, the easiest route is through Kolkata which is a 10 hour ride. However, if you are willing to rough it up a bit, both these places are definitely worth a visit.  Apart from these two, I will be running you through the other more feasible destinations of interest.

[NOTE: It is to be kept in mind though, that although the North East seems to be a pretty small region on the map, it has the most complex circuitry of roads and mountains that takes rather a long amount of time to cover even small distances. For instance, the Guwahati - Shillong route is a little over 100 kms and it takes over 3 hours to cover it when let's say the driver pushes it real hard. And the route is one the better ones in the region.]

on Shillong-Gauhati road (Source: Sankarshan Mukhopadhyay) 

Your best bet would be Darjeeling and Kalimpong. They are about 3 and half hour's drive from Jaigaon. And who has not heard of the 'Toy Train' and the world renowned 'Darjeeling Tea'? Until recently, they were the very identity of the entire North East although it has never been officially a part of the land, being a district of West Bengal. But apart from these world icons, there's a lot more to Darjeeling.

Darjeeling med mælk (Source: Darjeeling med mælk)