I live at a somewhat higher altitude than where you are going (2200 m) in a pretty pollution free area. The sky is not usually a problem, although it can be very blue. A yellow filter is not a bad idea most of the time, but it may sometimes look artificial with some of today's films. You can always look at the scene through the filter and judge it if in doubt. I usually pack a small bag of filters (8, 11, 15, 25, polarizer) and choose whatever I need for the occasion. If I were getting new film, XP2 would probably be my first choice. It is fine grained, forgiving, and produces spectacular images. If you are just bringing stuff from your stock, FP4+ would probably be first choice, especially if you are using a tripod most of the time. If you will be shooting in marginal light or be using the camera handheld a lot, HP5+ would be preferable. It still has very fine grain and has excellent tonality.