The Langtang Region

Beyond the aforementioned "Big Three" trekking regions of Nepal, Nepal is basically a virgin territory for trekkers. Trekking off the main paths is not only possible, but can be immensely rewarding, though you need a sense of adventure and an increased ability to deal with the unexpected. The treks range from teahouses to wilderness hikes. Frequently they combine both aspects by crossing over one or two uninhabited passes.
You need extra time to get beyond the standard routes, however, as said rewards are great - not just mountain views, but increased contact with a wide range of Nepalis, and the chance to glimpse a completely different way of life.

Dolpo
The best known of the many isolated high Himalayan valleys across the northern Nepal, Dolpo preserves one of the last remnants of traditional Tibetan culture. Legend says it's a bayul, one of the "hidden valleys" created by Guru Rinpoche as a refuge for devout Buddhists in troubled times. Surrounded by high mountains including the Dhaulagiri massif to the southeast rand cut off by high passes closed by snow half the year, Dolpo's easiest access is from Tibet, where its' people emigrated from perhaps thousand of years ago.
Upper Dolpo shelters about 6,000 people, whose lives revolve around Buddhism, barley, and yaks; their villages (over 4,260 meters) are among the highest settlements on earth. A large portion of Dolpo has been set aside as Shey-Phoksumdo National Park, at 3,555 sq.km. The park shelters blue sheep, Himalayan black bear, leopards, wolves and the elusive snow leopard.
Largely thanks to "The Snow Leopard" book and Oscar nominated movie, "Caravan," Dolpo is the best known of Nepal's remote northern border regions. One needs to get trekking permit from Department of Immigration in Kathmandu or Pokhara to visit this fascinating region. Check out travel faq for more information on this.
Phoksumdo lake at 3,627m is the most fascinating part of the whole trek in Dolpo. The lake is a basin of unearthly turquoise blue ringed by rocky crags and forest, framed by snowcapped peaks.
Rara lake, the largest lake of the country, is a major destination among the treks in western Nepal. The lake, located within the Rara National Park, is perched on a high shelf, encircled by gray ridges and pine forested hills inhabitated by beers, jungle cats and deer. The trail leading to the lake was built as a horse trail for His Majesty King Mahendra's 1964 visit to Rara.
Access to Rara Lake is from Jumla, which can be reached by flight or by walking for around ten days from Surkhet in western Nepal. A trip to the lake and back to Jumla takes just about ten days.