The easiest route is the Caps Ridge Trail, that starts from Jefferson Notch at an elevation of 3,008 feet (the highest trailhead in the White Mountains), which at 2.5 miles (one way) and with only 2,700 feet of elevation gain is the easiest route to any major peak in the Presidentials. It is steep, rough, involves scrambles over ledges, and has what some may consider an uncomfortable degree of exposure (a very individual assessment). In spite of that, it is definitely the easiest route for those comfortable with moderate exposure.
All other approaches are longer, with substantially more elevation gain. Probably the easiest way is to go up the Jewell Trail, from the Base Station, to the Gulfside Trail to Jefferson (5.5 miles one way, with 3,750 feet of elevation gain). It is nowhere very steep or rough, but is exposed to the weather for a long time.
From US 2 to the North the most common approaches are by the Castle Trail directly to the summit (5 miles and 4,200 feet one way), or by Lowe's and Randolph Paths and the Gulfside Trail (5.5 miles and 4,350 feet one way). Both involve some steep sections, and the Castle Trail involves some scrambles on the Castles.
A good loop can be made by going up the Castle Ravine Trail (5 miles and 4,200 feet one way) and returning by the Castle Trail. The final climb out of the floor of the ravine up the headwall is a very steep scramble on very loose rock. Descent by the Castle Ravine trail is not recommended. The lower end of the Castle Ravine Trail is largely *in* the stream, and should therefore be avoided in winter or spring. See photos #42 - #44 and #51, #52 here: Jefferson.
Another more difficult approach is from the Great Gulf by the Six Husbands Trail. Take the Great Gulf Trail to the Six Husbands Trail, which ascend the very steep Jefferson's Knee, with scrambles and two pairs of ladders (7 miles and 4,300 feet one way). It is not recommended for descent, the most common way to do a loop is to return to the Great Gulf Trail by following the Gulfside Trail south to the Sphinx Trail which descends to the Great Gulf. This is a long day, many people camp in the Great Gulf to split the trip over two or even three days. An alternative is to spot a car at the Caps Ridge trailhead in Jefferson Notch, this makes a one day trip quite doable.
One Day Presidential Traverse
See description on the Mount Adams page.
The Jewell Trail trailhead is located on the Cog Railway Base Road about 1.1 miles from the four way crossing with the Mt. Clinton and Jefferson Notch roads.
The Caps Ridge trailhead is near the hight of land on Jefferson Notch Road. From the south that road starts at a four way intersection directly opposite the Mt. Clinton Road, both going off the Base Road which goes to the Cog Railroad. The north end is on Valley Road in Jefferson, which runs between US 2 and NH 115.
For parking at Lowe's Store a $2 per day fee is charged.
The Randolph Mountain Club (RMC) has four shelters in the area:Log Cabin, Gray Knob Cabin, The Perch and Crag Camp (see map for their locations). Fees are charged at all of them (follow the link for each shelter), and no reservations are accepted. All four RMC facilities can be used for overnight trips to Adams and either Madison or Jefferson.
The WMNF maintains a group of primitive (no platforms, no outhouse) camping areas along the Great Gulf trail and some of the trails that branch off it. They are shown in the new AMC map and in the RMC map. There are two sites on the Wamsutta Trail just after it leaves the Great Gulf Trail, and one on the Six Husbands Trail just before the Buttress Trail branches off it. These sites are very useful for those attempting Jefferson by the Six Husbands or Sphinx trails. There are no fees, and reservations are not accepted.