Not sure of your NH resume and pace, but Mt Jefferson via Caps ridge is a great bang for the buck. Washington, Madison and Adams are all pushing (or exceeding )your 6 hour limit. Monroe from the Ammonosuc Ravine trail has some great views plus you get to check out Lakes of the Clouds.
Cherry mountain, including Martha's Mile has some great views. (probably on the low end for time)
Waumbek and Starr King is a great late fall hike, the views really arent that great until the leaves fall, but its a super well graded trail.
Jackson and Webster cliffs is a great day and its a loop to boot.