There are some instruments that are well suited for sitting around the campfire, as well as storing in your backpack:
Cowboys played them a lot, and for good reason — they can be put in a pocket or anywhere else, they're rugged, and with some practice they can sound rather nice. Difficulty to learn: 2/5.
They're small, rugged, very cheap, and easy to learn. When you go above their intended range, they're very shrill, and useful as a signaling device out on the trail. Difficulty to learn: 1/5. They have a reputation for having a poorer-quality tone, but there are a few people who play them quite well.
Used by schoolchildren because they're rugged and relatively cheap. They sound better than a penny whistle, but are a little more difficult to learn. They're quieter than the penny whistle and flute. Despite their usual association with schoolchildren, there are a decent number of more serious players.
keyless simple system flute
Used for military signalling, as they can be heard up to 3 miles away. Basically a small keyless flute, but more difficult to play.
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.