I'll make a suggestion here that will probably be completely out of line with what most others will suggest.
Demonstrate the simple efficiency of what your art contains. Don't go the showmanship route. Don't make up flashy forms, and don't emphasize the crowd pleasers like gymnastics and high kicks and board breaks and unrealistic and lengthy fancy combos. Instead, focus on the true root of the art and the efficient brutality that it holds.
This isn't to be morbid or to shock or offend the audience. Rather, it's to show what the arts really hold, and maybe talk about the history under which they developed the way that they did, the need for a truly decisive method of defending oneself.
Demonstrate traditional forms/kata, if your art has them. Don't fancy it up. Then, demonstrate serious bunkai out of those kata. You can do attack/defense scenarios, but make it real. The defender should defeat the bad guy with quick and nasty kicks to the knees, pokes to the eyes, painful joint attacks, throat attacks and stuff. Don't kick 'em in the head unless you put him on the ground first. If you do any board breaks, keep it simple and emphasize that this is simply a method for gauging the power that a given strike holds. Don't get fancy with it. Give them a glimpse of "this is true martial arts", not flashy babysitting and entertainment for the kids. Make the audience feel like they got to peak into a hidden world that most people never see, the world of real martial arts.
I think it would be a nice change of pace, compared to the Hollywoodized demos that are often done at the local shopping mall and stuff. It could be pretty cool... I'd have infinite respect for you, if you did this