I stayed in Interlaaken at Christmas and it was great. The Jungfrau region is nothing like you've seen before. 19 days is plenty of time for where you want to go. 3 days in ams tops, 1 day in brugge, 4-5 in paris - you can fill in the rest.

In the winter Interlaken makes a great choice as higher climes/climbs are a lot colder and even streets in those mountain villages can be icy and slippery (as they were once in Murren in mid-September several years ago.