The Jakobshorn only has routes from the midstation (Jschalp), there is a good singletrack route to clavadeler Alp, which then goes down to Clavadel on a gravel road, then head back onto single track and gravel to Davos Platz.

From the Parsen, you can get off at the Middlestation and do the Panoramaweg singletrack - towards Klosters is best, at the Parsennhutte you can head down towards Wolfgang or across to the Schwarzseealp and then back to Wolfgang.

A fairly difficult route can be done from the Weissfluhjoch, down the Hauptertalli Lots of rocks, then very narrow singletrack), over the strela and back along the Panoramaweg to the Parsenn middle station.

The rinerhorn is definately worth a mention, there is a great route from the top station via Oberalp to Monstein (awesome singletrack), then back to Glaris (Rinerhorn station) you can extend by heading from the Spina restaurant to Junkerboden and back to Davos Platz (some singletrack).

There is so much biking in Davos, Ive been here 3 years and not even scratched the surface.