I've driven my car around Maghalaya a bit, very beautiful and enjoyable and a great break from the usual intensity of India. Cherapunjee is an lovely little place (in the dry season!), and the road from Shillong is stunning and empty. I'd highly recommend trekking a little bit down from Cherapunjee to see the root bridges in the jungle.
Tripura was also nice, though there were convoys which one had to join. Lots of tribes, though not as untouched as those in Bangladesh. Very friendly people.
I went through Assam a little, though aside from seeing the Brahmaputra is was no so exciting. Plus there is a lot of trouble there at the moment, which may restrict access. In my experience in India, Assam has the worst roads.
Unfortunately I did not get to any of the other NE states as permits were required at the time. Mizoram is meant to be very beautiful especially south of Aizawl.
Nagaland is also meant to be very ethnically interesting.
I am not sure if you'll get into Manipur as it was basically a warzone a couple of years ago, and may still be for all I know.
Sorry not to be of much help. I'd be very interested to read about your experiences there. One thing to bear in mind though; it's all near sensitive borders, so it remains to be seen how much of the outer NE states you can visit.
With the de-restriction of NE states to foreigners (they are still restricted to Indians I believe) and political reforms in Burma, we might just see that border open within our lifetimes.