Highwater Trail Loop, Bethel
Starting from the Hasting Plantation parking area on Wild River Road The Highwater Trail "Loop" offers two completely different riding experiences.
On the South side of the river you follow a graded dirt road, Wild River Road until you get to a bridge across the river at Moriah Brook.
Crosing Wild River there you return on a difficult singletrack trail, the Highwater Trail, with more than a few hike-a-bike sections. This should be attempted by strong experienced riders.
Do both and you'll be riding for about 15 miles. Although if the Highwater Trail is too difficult you can shortcut the hard stuff by crossing the river. (That's best done in the summer when the COLD water is lower.)
After you cross into New Hampshire you'll be in the White Mountain National Forest and indeed Wild River Road leads to Wild River Campground where one could camp if they desired.
This ride, on either side, of the river is very scenic. You'll probably see no one save for a few vehicles on Wild River Road or the occasional fisherman. I suggest bringing extra food and plenty of water if you try to do both "sides" of the loop.