Nepal has many trails to ride on. You can make a challenging climb thousands of metres to reach a viewpoint with fantastic views. Then you make an exhilarating descent. It is an interesting trip ride to Royal Chitwan National Park.
The long bike tours require that you are physical fit. Unless you plan to ride along a trekking route, there is no required paperwork. To do any serious biking you need a mountain bike, and a normal bike will not do.
The Kathmandu Valley has some great places to ride. It has many trails and backroads. It is a good way to visit the interesting towns, temples and Buddha stupas. It can be really interesting visiting a rarely visited traditional Newari village. To get around the valley you most likely will want a good map, such as the Schneider or Nelles Verlag map. Most of the other maps are not very good and are often inaccurate.
Many trails are not on maps and you will need a good sense of direction and will have to stop and ask directions to get around. A little Nepali can really help.