It’s endless. We’d be here for days describing it if we tried.
Buy an IGN map when you get there and go exploring. The Super U and Intersport on the way out of BSM towards Val D’Isere sell them. There’s excellent stuff linking the villages in the Ponthurin Valley (the valley in between Les Arcs and La Plagne) back to the ski resorts on both sides like Montchavin, Les Coches, Vallandry and Plan Peisey.
A surprising amount of what seem to be sneaky walkers trails are marked on the maps and quite accurately reflect what is on the ground. Use the local buses with bike racks on the back, the funiculaire and the ski lifts if there in July or August to get up high early and explore to your hearts content.
I guided there for two seasons then took the wife back for our honeymoon two years ago, I’d say I know barely 10% of the stuff. Enjoy. It’s an incredible place.