Col de Cou
Trail that has now entered mountain biking mythology.
Easy to reach: from Chalais you take the cable car to Vercorin: you pay on arrival and there you also make the ticket for the next one that will take you to the Crete du Midi. To reach this you have to pedal about 1 km through the village (signs).
From the Crete you go down a nice path to a dirt road that you follow to the left. There is very little pedaling, because once you get to Le Pichioc you will have to lug your bike up 350 meters of elevation gain to the Col de Cou. That's enough from here. It's all downhill all the way to the valley. 2000 meters of elevation gain where you don't need to give a pedal stroke or get off the bike once that is one.
Once in St. Leonard you pedal quietly to the starting point, the cable car parking lot (free).
Very smooth trail, beautiful. Tour that can be done in half a day, if you don't stop for various siestas.
Chalais-Vercorin cable car: http://www.chalais.ch/commune/chalais/valais/horaires.html