questionswhat would be a highly portable, fun instrument…

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Harmonica. You can play it with or without the guitar.

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Harmonica. Or, the spoons. You can eat when you're done.

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Since you play piano and guitar, the logical progression would be:

The Keytar, particularly this one.

Otherwise, in addition to the other posts (except for kamikazeken's...), I would recommend the kazoo, vuvuzela or portable xylophone.

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the flute, but played like Ian Anderson of Jethro Tull

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Another vote for ukulele. They are ubiquitous here and definitely lighten the mood no matter what kind of day you are having. Unless you are partial to sticking with your favorite guitar brand, why not try out one of the Hawaiian makers? (Makala, Kamaka, Lanikai) There are many and they have a range of pricing/sizes and of course beauty.

As for something a bit different, but stringed, there is the banjolele.

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@kamikazeken: I was going to say the fleshtone trombone

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@kophia: Thank you very much for the detailed response! And thank you to everyone who answered. I will take all of the suggestions into consideration. I'm still leaning toward a uke, but maybe I'll end up getting more than one instrument if I can find them for good prices.

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?@mkentosh: ooooh, how bout the rusty trombone?

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A penny whistle or tin whistle is a great, incredibly portable instrument. Works especially well if you're into Irish or folk music.
Doesn't have strings, so you'd have to learn the fingering, but it's not terribly difficult.