Kancamagus Highway is definitely at least a full day trip. Anything less will not give you enough time to enjoy many of the sights. We made it a 1-day trip from Weirs Beach and really got only a taste of the highway, missing many of attractively named trails and sites.
Kancamagus Highway is an official scenic byway. As it climbs over the slope of Mt. Kancamagus, it reaches an elevation of almost 3,000 feet. Its full route, Conway to Lincoln, was completed in 1959.
The visitor center is located on the highway's east side, so you may want to drive it from Conway to Lincoln rather than the opposite. We had a detailed map of the highway with the overlooks and trailheads from our resort, but I am sure the visitor center would provide one, too.
For some parking areas, you will need to purchase the White Mountain Pass, currently priced at $3 for 1 day. Longer-term passes are available. Check the USDA Forrest Service Site for more details.
We had time only for a few stops.
(1) The covered bridge at the highway's beginning was number one. The sign reads Covered Bridge Campground. That is where you need to turn. We like covered bridges, so this was an automatic stop.
(2) Lower Falls was our number-two stop, shortly after the covered bridge. You will clearly see it from the road on your right. These are nice, small, easily accessible falls.
(3) Rocky Gorge Scenic Area was our number-three stop. It's a rocky riverbed with marked areas of permitted swimming. It was too cold to enjoy that, but I can see how great it must be in mid-summer. Only in the pool under the waterfall swimming is prohibited. There's also a nice wooden bridge over the gorge. Again, this is only a very short distance from the parking area and extremely easily accessible.
(4) Sabbaday Falls was our fourth and final stop along the highway, and it was well worth it. With this one, you actually have to walk a little bit to get there, but I think you won't be sorry you did it. There's a nice cascading waterfall where the bottom portion is at 90 degrees to the upper portion, making it special. If you like taking pictures of waterfalls, this is a great site. There are wooden walkways and stairs around the falls.
As an added bonus - there was a black bear at the final descend to Lincoln eating dinner just by the roadside. Again, my camera was firing. Being in New Hampshire, what else could we ask for? Yeah, I know, a moose! We got that one the very next day, but that is a whole different story.