I see an interesting old thread has come back to life.
I perform at a variety of outdoor festivals and through that have come to know many other musicians who have an interest in outdoor, acoustic performance. As a rule we have to carry our instruments all day so as a community we have over time figured out some things that do and don't work.
The smallest, lightest, and most durable instruments suitable for performance are hand percussion (shakers, tambourine, finger cymbals, etc) and the "Irish" whistles. There are a number of better quality sets of whistles out there and a few boutique makers who produce sets with a single fipple and interchangeable bodies to cover a variety of keys. It takes at least three whistles to cover enough different keys to be useful for performance.
Next on the list are mandolin, violin, and flute, which aren't as durable but are still small and light. We find that the mandolin is the smallest instrument that can, by itself, provide useful accompaniment to a singer.
While the guitar is a larger instrument there are backpacking guitars that work out OK, and a few of my friends use them for public performance. My road guitar is a cheap Ibanez cutaway that has a built in pickup and tuner, which means I don't have to buy new tuners all the time (they inevitably get lost once in a while, and more often if you're a street performer). I have a tuner app for my android phone but it isn't practical to pull it out in the middle of a show.
The accordion is the most portable of the keyboard instruments. Newer accordions are typically smaller and lighter than those from the "golden age of the accordion" (1950s). Mine weighs about 15 pounds. The stylistic range that can be achieved by a competent player on a good instrument is considerable. They're not just for polkas and waltzes.
I have a stage piano and while it is portable and can withstand the rigors of the road it's big and weighs 40 pounds, and I don't usually bring it along unless I have a gig that absolutely requires it. It requires a stand, amp, speakers, stool, cables, and so on, although I guess there are consumer models that are self-contained and have little speakers built in.
I have trouble recommending some of the common folk music store instruments that lack expressive and stylistic range because it still takes work to learn to play them but they can't be made suitable for performance. The psaltry, mountain dulcimer, and autoharp are all in this category IMO.