There are some instruments that are well suited for sitting around the campfire, as well as storing in your backpack:

harmonica

penny whistle

recorder

keyless simple system flute

fife



For hikers who want to use their instrument as a signalling device — a note on loudness: Flute-like instruments are louder if they have a wider bore (larger diameter). Any flute-like instrument can be played much more loudly by going to a higher octave, though this can get pretty difficult on flutes that don't have a fipple.