I do understand that you are looking for sightseeing but some of the skiing towns are particularly spectacular and offer other things than just skiing such as sledging or ice skating.

The Rheinfalls near Schaffhausen are very impressive especially after it has been raining.

Going out to Appenzell and into the hills is quite bracing if you enjoy walking in the cold and "popping" into a small local "cafe".

As far as cities are concerned, Zurich, Basel, St. Gallen, Neuchetal, Bern and Luzern are all worth a visit as well as any on Lake Geneva. Going to some of the museums is also a possibility along with actually going up the mountains to walk. Actually heading south while pretty is generally not recommended "in season" as the routes are limited and the traffic heavy.