I think that there should be an exhibit on fractals and a history of mathematics exhibit. Fractals are very beautiful and mathematically interesting, and the concept of self-iteration is fairly easy for an general audience to understand. You could even discuss the motivating problem of determining the length of the coast of Britain and how making measurements of non-smooth curves on smaller and smaller scales eventually limits to infinity. As for the history of mathematics, I think that's pretty obvious. There are a lot of interesting stories that make up the history of mathematics. Museums also love to show cultural diversity, so the development of mathematics around the world could be a possibility.