We live in the age of mobility: we expect to have GPS maps of the world downloaded into our smart phones, to e-mail clients from a beach in the Caribbean just as well as a beach, and we expect to take it all with us in one small, simple place.
Oddly enough, much of our music gear has gone the opposite way: synthesizers and studio equipment can't exactly be hauled away in a purse.
So if you want to produce a sound from anywhere, you'll need a "mobile" instrument, just like they had in the good old days.
So which are the best ones?
The harmonica is one of the great pocket instruments, and there's a reason the familiar sound is so frequently associated with frontier life: it's so easy to bring along! True, the harmonica doesn't have a whole lot of range when it comes to producing timbre, but sometimes you have to sacrifice sound for mobility. How small do they get? The smaller harmonicas can look more like USB flash drives rather than musical instruments.
The acoustic guitar.
True, the acoustic guitar is lunkier than a harmonica, and it won't exactly fit in your pocket, but it's still a simple instrument that can produce a familiar sound that people love to listen to. There's a reason it's a favorite of street musicians. They're usually light-weight, fairly easy to learn, and you can tune them yourself. No batteries required! A kazoo is more "mobile" than an acoustic guitar, but there's a good chance people would rather listen to the guitar.
Okay, bear with me. The accordion is essentially a wind-powered portable keyboard, and it's hard to find a keyboard that you can bring with you. You can't exactly lug a piano with you on vacation, so if you want to satisfy those itchy piano fingers, there's nothing for you to do. Unless you bring an accordion. Accordions can be carried around and even danced with, if you're into that whole "polka" scene.
Similar to a flute, the recorder is actually slightly more mobile because it doesn't have to be carried in a case - you can simply leave it somewhere (preferably clean, of course) and pull it out when the time is right. It's also easy to play - not quite as easy as the kazoo, but it sounds a lot more pleasant.