harmonica

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.

penny whistle

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.

recorder

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

The keyless ones made of PVC are relatively rugged. They're more difficult to learn than any fipple flute, but they have a more pleasant sound.

fife

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.