Helen, Les Arcs is somewhere I'd love to go back to and in my opinion, the best runs are in the 2,000 area but as Fiona said, the cable to the summit at 3,226 metres can have really big queues (I only bothered twice in a week).
Anyway, I'll have a go at some recommendations
If they go up to the summit, there are some great routes off into the bowl below, probably Genepi and Robert Blanche being the best two and of course, the namesake run, 'Aiguille Rouge'. You don't need to go all the way up to the summit to enjoy most of that mega long run down to Villaroger (with its views over to Sainte Foye and La Rosiere) because there's a chair from Arc 2000 to the col a bit lower down that's barely used. The wildest and prettiest (and often ungroomed runs) are also in Arc 2000 except for Malgovert (just above 1600) which is a stunning and slightly mad run (think of a winding Ciste Gully with trees and bumps). Droset is a run off the col shortly below the chair, down through quite wild terrain and which crosses the road to the village. Comborciere is a another very pretty, remote and quite gentle couloir which gets bumped up from top to bottom as it winds down through scrub and trees from the lift above 1600, down to 2000. Refuge at Grand Col really feels like big mountain terrain, sitting under one of the tower block sized buttresses off Aiguille Rouge. Lastly, Bois de L'Ours is a steeper route down into Arc 2000 area on the main drag in from 1650/1850 which avoids the crowds.