Itanagar is completely different from the other state capitals I got to see on my trip to India. It basically lies boxed in by the surrounding mountains, which makes for a very verdant setting. You could well describe it as an XXL-size version of the little town of New Ziro (Hapoli) in fact.
As well as the various facilities and services the city offers (cash withdrawal, internet, shopping, etc.), Itangar also has a number of interesting places to visit. To begin with, there's Lake Ganga, a small lake that makes for a good area to go picnicking. Then there are the ruins of Ita Fort and the traditional village of Poma, with a visit to the latter providing something of a foretaste of the state's central valleys.
I particularly enjoyed shopping at the market in the town, a bustling, lively place to which people from other regions flock to stock up on exotic products. Though some of the tribes of Arunachal Pradesh are apparently very fond of them, I didn't actually see any rats or insects on sale myself.