Dressing for the Alps?

I am graduating college on Saturday, and my graduation present to myself is a one month tour of Europe. I am planning on visiting many countries including the Germany, Netherlands, Belgium, France, Spain, Switzerland, Italy, and Greece. Since it will be mid/late May and early June, the majority of my trip will be rather warm, and that is what I am packing for. The problem is the two days I have planned in Interlaken, Switzerland. I am doing the proverbial "backpack," so there is not a lot of room for extra items. I don't know what to expect in this region of the Alps, and I don't have the room for really heavy clothing since I need to focus more on the other 27 days. I am also planning on trying to go Parasailing, and it is the first time I have ever done anything like this, so I have no idea what to plan for. So, my question is, what can I do to prepare for my short time in this beautiful area?