I spent a week walking and biking based in Les Houches at the end of the summer season in 2008.
One day we got the cable car from Les Praz up to Flegere, then the chairlift to L’Index. Was a lovely day but damn cold up at 2385m! From L’Index, it’s about 4km over to Lac Blanc, which also has a refuge if you want a drink. We looped back and down to Flegere but there’s loads of options if you want to walk all the way down into the valley. Google turns up a couple of route guides eg this.
Another day we grabbed the Bellevue cable car out of Les Houches, then followed the Tramway du Mont-Blanc up to station at Mont Lachat. You can find a nice spot for lunch round here, with views straight up the valleya and it’s a popular launch spot for paragliders. We found a path which was waymarked and took us all the way back down to Les Houches.
You can find the summer lift opening times online eg Flegere / Brevent. One thing to note is that Compagnie du Mont Blanc operate a number of the lifts but not all. The Bellevue cable car out of Les Houches is run by a different company.