Nepal has much more trekking and adventurous destination. Some are allowed to visit and some are restricted to travelers. In the restricted areas, travelers need to book the trip through the trekking agencies of Nepal those who are registered and affiliated with the Tourism Board. Most of the northern parts of Nepal are restricted due to the consideration of saving the environment, culture and the mountains over there. Nepal has the borderland in the northern part of Nepal, for the tourist safetly, areas are restricted only for solo travelers. At least two travelers need to be in a group to make it happen.
To issue the special trekking permit must be acquired from the Department of Immigration, for trekking to areas that fall under the restricted zone. For more updated information you can check this official website: http://www.nepalimmigration.gov.np/post/notice-regarding-visa-fee-updates