You'll need everything. Sugar is a FIFO, starches for the midrange, and fat for the night. Trick is nothing with a high moisture content...

Granola bars. Peanut butter cookies (good luck keeping them in the pack) or sandwiches (they can keep in the pack for several days; weeks if frozen). Sunflower seeds. M&M's can chip teeth if they freeze, so if you do gorp, use plain chips. Fig newtons are great.