Together with my wife we are thinking of revisiting Switzerland after 6 years, this time in the first 2 weeks of December 2015. Last time we went, we had Geneve, Interlaken, Lucerne and Zurich as a base, and it was September, then we took some day trips from there using trains. We are thinking of either using the same bases and visit some different places, or else staying in different cities. What other cities are a good base that we can do easy day trips using trains and also with some Christmas spirit? Zurich must definitely be there as a last base to fly back to home. For the first base I need someone that I can easily reach either from Munich or Milan with train not too long, else Geneva? I'm ready to use the same bases as I did when we last visited. I can do 3 bases instead of 4. Btw, we like scenery, museums and architecture
I've only been to Switzerland once, this past September. But I would have been very upset if one of my bases hadn't been in the Alps. Not near the Alps, like Interlaken, but in the Alps. I stayed in Mürren and loved it. Other options in the Berner Oberland are Gimmelwald, Wengen, and Lauterbrunnen. Each have their adherents and detractors, so read more about them before deciding which one would be best for you.
The Berner-Oberland is easy to reach from Milan via Spiez and Interlaken West. We were there for New Year's 2014 and loved it! If you stay in Lauterbrunnen, in the valley, you may not have snow, but there is easy access to the mountain towns on both sides of the valley via the extensive network of trains and lifts. You can either just ride and enjoy the scenery, or take some magnificent winter hikes on the groomed trails. You can also stay up in Murren or Wengen, for example, and be in the snow, still with access to all the trains and lifts. (Buy a pass!) FWIW we found Lauterbrunnen convenient as could be as a base for our adventures. If you stay in Wengen or Murren (where we stayed in fall of this year) it is just a bit more of a trek to go to the other parts of the area. i.e., Murren is a bit of a long trek from Wengen and vice versa, but Lauterbrunnen is in the middle. We found 4 nights to be a good stay.
I'm glad that you plan to have a few home bases. (As good as the Swiss public transportation connections are, it's not really practical to see everything from one central location.) Have you seen Bern, Lausanne, or Appenzell? For visiting the Berner Oberland, I see no benefit to sleeping in Interlaken. The beauty of the Alps is best experienced by waking up in a small, quiet town with a mountain panorama right "in your face," as you might experience it in Gimmelwald or Muerren.
We had Bern as a base and it was easy to do a daytrip to the top of the Schilthorn and another daytrip to Lucerne. And it's a lovely little city to boot.
Geneva is way, way west (virtually inside France) so that can limit your access to the Alps unless you take the Golden Pass to Spiez or Interlaken.