For a day you can ride the marked VTT trails and add in one or two other descents. The big lift up from the valley in Les Chables runs all day (easy to park there and get lift up then you can finish with a big descent). As does the one servicing the bike park trails. Note the Tzoumaz lift shuts over lunchtime. The runs down from Savoleyere to Tzoumaz through the woods are a bit more unofficial but good. Follow your nose/tyre marks. I found Vallon D’Arby a bit boring (probably as I failed to ride the rocky technical sections !), ride from lift at top of bike park around towards Croix de Couer/Savoleyere, climb 150m to Col des Mines then follow path left. There is a good descent to valley floor from top of bike park round to La Chaux then down. You can ride down from Verbier village under Les Chables cable car past a small church. The swiss paths are well marked with signs
Stuff can be pretty steep there ! The Red bike park trails are quite lively, I didn’t try the blacks.
Have a look online at maps here Online Maps
If you drop me an email I will try and send you a map with some of this marked, sorry no GPX
You could also try these guys – I think they are same side as you will be (ie Nendaz/Sion) – bestrails.ch their quoted weekend prices are rather high
[video]http://vimeo.com/83843763[/video]