The Martin backpacker guitars and mandos are very "lashable". They are also tough enough to use as a weapon....and sound similar to guitars and mandos. The gig bags they come with protect the instruments well and you can simply lash through the hand hold with some bungee cord, lengthwise along the pack somewhere. I have the mando and it's been in kayaks, airplanes, on mountain tops, etc. I wouldn't recommend a full size dreadnought or even a OOO or concert instrument unless you are hiking to a fest and need a good instrument.
I've also been known to bring along penny whistles, Irish flutes, and harmonicas. Much easier to haul.