Easy. Recommended for families with young children or less experienced cyclists. No traffic.
From Beitostolen, drive to the North on road 51 to Bygdin. After you have passed the Bygdin Høyfjellshotell, turn left and find a place to park your car. From there, follow a nice mountain gravel road along the Bygdin Lake for approx. 3 km until the road ends, and return on the same way. The scenery in this area is breathtaking!
If you want a more challenging route, start cycling from Beitostølen. However, due to the traffic on road 51, this is not recommended for children.