Most of the items here will of course be focused on supporting your hiking performance, but a big part of trekking life is what to do during down-time in the evenings.
Teenage Engineering's Pocket Operator (PO) series of mini synthesizers place a whole world of creative possibilities in the palm of your hand. About the size of a calculator, each unit in the series (I purchased the PO-12 for £49) offers a plethora of programmable taps, beats, rifts and rhythms to keep you entertained long into the night.
It looks complicated at first but don't let that hold you back - once you play around with the controls you'll be stringing together a 16 stage sequence as DJ Himalaya in no time. Once you're back home, combine with a full size synth and a laptop to orchestrate a mountain inspired mix.