OverviewGreen Mountain from the sky looks like an isolated outlier of the Ossipees, to their east and near the Maine border. While not tall, its isolation provides excellent viewing from its summit Fire Tower.
The most popular Trail is the High Watch, from the north - about a 3-mile round-trip with 1,200 feet of elevation gain. While pleasant enough, it is a bit eroded and there are no views before the Tower.
My favorite Trail is the Dearborn, from the SE - just a little longer and more elevation but a lovely walk through open hardwoods, with some ledges higher up. The Society for the Preservation of New Hampshire Forests purchased the land in 2007, so now there is a trailhead kiosk and clearer blazing.
Thr Libby Trail from the SW is less accessible and is eroded from ATV use. It was also still closed as of Sept. 2008, from damage from the July 24 tornado, which spared the other two Trails. (I attach a few photos of that damage.)
For great viewing off the beaten track in central NH, it is hard to do better. This summit deserves its place in Scudder's White Mountain Viewing Guide.
Getting ThereHigh Watch: From the junction of NH 25 and NH153(northbound), near Effingham Falls, take Green Mtn. Rd. south for 1.2 miles, then left on gravel roads, following signs for "Green Mtn. Fire Tower" and/or "Mountain View Rehab. Center." There is a clearing with plenty of parking at the trailhead, just beyond the Rehab. Center (which is a huge and eerily quiet facility for patients with serious neurological injuries).
Dearborn: From NH 153 just north of Effingham Village, take Hobbs Road west 1.2 miles to parking area with kiosk on right. Hobbs is a dirt road marked by a green historical sign - "First Normal School in New Hampshire."
You can find more details in the AMC Southern New Hampshire Trail Guide.