I ski, snowboard, and hike, so when I'm really going to be working hard at high altitudes, I like trail mixes, which are nuts, dates, figs raisins, cranberries, dehydrated berries. Dried fruits are good. Also granola.
These mixes are available commercially but can be pricey. They are easy to mix up yourself and carry in ziploc bags.
Take lots of water. I use a Camelback.