Lauterbrunnen Valley in early May 2002

I'm female (and very cold-natured), which may make a difference here.

Layers for sure. You'd potentially need them even in the middle of the summer because altitude really affects temperature.

I was glad to have 97% nylon slacks (PrAna, Columbia, etc.) for Andalucia in April and Scotland in the summer--places/times where I feared chilly, wet weather. The slacks weren't waterproof but were pretty good at shedding rain. With long johns underneath (merino wool in this case; sometimes I use polypropylene) I never got chilled to the bone. Those slacks really made a difference.

I always take a waterproof rain jacket; my trips are too long to drag a full-length raincoat around, but on a 21-day tour I might change my mind.

I take waterproof shoes on any trip with a substantial amount of time in potentially rainy areas--which would be just about anywhere in Europe in April.

Some sort of gloves would be a good idea. You might not need them, but they won't take up much room. I'd also consider a warm hat. I always travel with a sun hat and have never really needed anything warmer on my head in Europe, but I haven't been to Switzerland before Memorial Day. I use one of those ear-wrap things when I take domestic trips in the winter; I hate cold ears.