I was in Switzerland last fall and travelled with a 5d3, 24-105, 16-35f2.8 and 70-200f4. That is my typical travel kit lately. My most used lens on that trip was definately the 24-105, but the 16-35 was a close second. Switzerland in particular has some amazing vistas which just scream for a UWA (or stitched panorama). Having said that, depending on your photo style, you can use pretty much anything to get good shots of the Alps and the 70-200 is great for getting shots of more distant mountains or scenes.

As for the f4, I've never had much issue with that being too limiting. I sometimes bring a flash or a tripod for lower light, but the high-iso capabilities of your 6d should help alot.

What areas of switzerland are you visiting? I know we have at least one Swiss member here (Pierre, aka Tounis) who gave me some advice before my trip...

Stephen