Traveling in early December

After Lausanne and area, consider Luzern as a second base. It is only 60-90 minutes from Zurich airport, and can be used to ascend Pilatus mountain.

Pilatus may close down in December, but Mt Rigi is open all year round, and is spectacular with fresh powder on a clear day. Lucerne is charming. It's Xmas markets open later in the month, but the Saturday farmers markets are fun.
Zurich main train station has the largest indoor Xmas market in Europe.
Lucerne is an easy commute to the Zurich train station - is the track work finished, fellow posters?
Are you actually staying in Geneva? Will you be jet lagged? So you have nine nights in country?
With a little more info, I could be more helpful.
Enjoy this beautiful country with a world- class rail system. Safe travels!

I've done 2 trips to Switzerland in December (one early, one mid-late). Christmas markets are my favourite, especially the one at the Zurich train station. Anywhere with skiing you will need a reservation (if the weather is mild, ski season will barely be getting started). We had a spectacular clear two days in Zermatt around the 10th of December, but it was definitely into ski season and I'm not sure what there is to do there if you don't do snow sports and it's cloudy.

Cities are generally at lower elevations so snow is possible but more likely fog and rain. If you find yourself in a foggy city, try to get up above the clouds. From Luzern we've escaped to Rigi (fabulous spa) and Engelberg. I think Pilatus is open all year, but only by cable car (not the cogwheel train)

Wherever you go, hotel reservations are recommended.

Enjoy the Christmas markets!