Based in Les Houches near Chamonix- day trips?

Consider visiting Annecy, it’s one of my favorite places in France ;-)

Have a wonderful trip!

I looked up Annecy based on your recommendation, and it is beautiful! We will be going to explore Annecy and enjoying the Sunday market! Thank you.

You can head to Evian (direct) on the Lac and catch a ferry to Lausanne, Vevey or Montreux. Also check out Yvoire, a gem of a village on the lake. You can also drive, of course. Also nearby are the Chateau du Chillon and the Lavaux vinyards. Play it by ear because if the weather is no cooperating with the lifts you can take these city/lake breaks instead. Already in September it can get a bit dicey.

If you are a hiker, there are number of great hikes over that pass, and on the way to the Swiss side, like at Vallorcine. The incline railway and view at Lac Emosson is terrific, with a bit of fondue d'alpage at the restaurant at the dam. If you head to the Italian side, there is a return route over the Alps from Aosta. It's a nice drive but not one you want to do in bad weather or at night -- and probably a pretty lengthy return for you. (I used to live in Lausanne and we had another office outside Milan)

We’ll have a rental car. Is there a ferry that will take cats across to Montreux or Vevay or another city? Would it be better to just use public transportation?

There is no car ferry, you have to drive around. The ferry is quite short and the public transport options on that side of the lake are quite good. You can take the metro from the Ferry area in Lausanne Ouchy and the ferries disembark in places like Vevey, Montreay and the Chateau du Chillon with at most a minimal walk. The view from the boat up the Valais is quite beautiful.