Trip holding a harmonica
The harmonica was a small metallic hand-held wind musical instrument from the planet Earth.
Commander Trip Tucker owned a harmonica and sometimes played it in his quarters to relax. (ENT: "Precious Cargo") T'Pol remarked that he would have plenty of time to play it when imaging relays were working automatically. (ENT: "Horizon")
Tucker traded his harmonica for a horse in the Skagaran colony. (ENT: "North Star")
The harmonica played by Commander Tucker appears to be a Honer CX-12.
Coincidentally, a harmonica played by astronaut Wally Schirra on Gemini 6A (along with some small bells rung by his crewmate Thomas P. Stafford) is believed to be the first musical instrument played in space.
The harmonica was, according to the Star Trek Encyclopedia, 4th ed., vol. 1, p. 327, "traditional to the style of American folk music known as "the blues."