The Tawang Valley is one of the most popular destinations in Arunachal Pradesh. Though not as impressive as the valleys of Ziro and Mechuka, it does definitely deserve its place in the top three.
When you're visiting India, one option you have is to plan your trip to Tawang to coincide with the festival held there at the beginning of May. Do note, however, that you will need to at least have an itinerary planned in order to obtain a permit, which you can apply for in Guwahati if you are not visiting the region as part of a tour organised by an travel agency.
Once arrived, you'll no doubt end up spending quite a bit of time at Tawang Gompa, which is apparently the second largest monastery in the world! Its library its quite incredible. I also recommend spending a least a day hiking around Tawang so that you can also visit some of the lesser known monasteries.