When I was preparing for my first cross country involving mountains, I got Oxygen (already mentioned) a pulse oximeter and a Personal Locator Beacon. My wife and I have found that our O2 needs are quite different and can tailor the flow based on the oximeter readings and the PLB is for peace of mind. I wear a fishing vest with the PLB, Oximeter and, possibly the most important piece of equipment,...
CAMERA
Have fun and take lots of pictures, including the panel so you can answer those performace questions about the trip that you never write down!