Hello Gavin,
Hope things are well for you. Some possible suggestions:
1) Display different martial arts uniforms.
2) Display various martial arts weapons, where they originate, their history, and perhaps a tape or live demonstration of how these weapons are used.
3) Various pictures, past and present, of martial artists in combat (I've seen some old paintings/photographs that were awesome! Some could be located on the internet.)
4) Sword displays - there are a lot of different swords used in martial arts.
5) Video clips of different martial arts/styles, showing how much of a diversity there are in the world.
6) Ask kids to make/draw what they think martial arts is about. They can use paper, clay, cloth, plastic, wire, etc., whatever they can use to be creative and display their creations. You'll be surprised what they come up with and how they view martial arts. Its best to ask the kids who are actually students of martial arts.
There are more suggestions, but these are just to start and maybe you might be able to come up with some of your own ideas.
Good luck.
- Ceicei