Chamonix is about an hour away from Les Gets I think - we met a few people who had come down for the day. The riding is fantastic - steep, tight, rocky and very technical in places - big tyres needed (I was using Nokian NBX 2.5DH), and armour is a good idea too. Oh yeah, a chain device would have come in handy!
The trails are marked on the VTT map from tourist information - all the 'official' trails are jeep track, but these are linked by walking trails not marked on the bike maps - fine to ride on, just be nice to walkers! These are often very steep and technical - take it easy...
If you're only going to be there for a day, try the Lognan meadow run where the picture was taken. You take the Grand Montettes lift from Argentiere (15 minutes on the train from Chamonix) up to Lognan, and take the jeep track signposted La Lavancher from the lift station. Go past the chairlift station, and up a short climb with a barrier at the top. Start descending jeep track again, the entrance to the meadows is on the left, on a right hand bend - it's pretty obvious when you see it. Descend through the meadows singletrack (ace!

), and exit at the lower right hand side (careful of a wire strung over the trail). The trail turns to jeep track again after a bit, descend this until you see a small steep track leading on a hairpin left hand junction (the fire road carries on straight down) - take the hairpin for steep switchbacked fun. The trail ends at La Lavancher - the options here are signposted. you can either descend to Chamonix / Les Bois (an absolute blast - fast rough jeep track, with water bars you can launch off), or head back up to Argentiere on the Balcon Nord, and do it all over again!
Have fun! What bike are you taking?