The Roaring Twenties were also known as the Jazz Age. Although jazz had been around for decades prior to this, with the advent of Prohibition and the creation of speakeasies which both served alcohol and often provided a place to dance to music, jazz became much more mainstream, at least amongst the younger set. The more established tended to denigrate jazz and the effect it was having on the youth of that time.

One option for a 1920s costume is, therefore, to dress up as a jazz performer. For singers, especially women, their appearance would be little changed from that of the day, and it would therefore be less easy to tell it was a distinct outfit. For musicians, there is always the musical instrument, as there were even women instrumentalists. Instruments strongly associated with jazz were the piano, some strings, such as the double bass, and brass instruments, especially the trumpet, trombone and saxophone. One of the brass instruments would be much easier to have as part of a costume than the piano, or the double bass, of course.
Dressing up as a jazz musician is a possible choice, or even as an entire jazz ensemble for a group costume. The instrument would be the primary focus of the costume, as without it would be hard to tell what the outfit was. The typical normal dress of a musician could be something like a suit and hat for men, or a flapper outfit for women. An alternative would be to dress up as one of the more famous jazz musicians from the era, such as Louis Armstrong, who are still known today.