I woulld like a math theme for a theme day in a high school, grades 9-12. Our last theme was fractal fest.

Here are three ideas.

1. Games theory provides a lot of examples of games with high mathematical contents, puzzles in general are related to mathematics, and even physical games such as pool and baseball can be analized with geometry and calculus.
   
   
2. The theme could be calendars, especially if you have an ethnically diverse class. You could look at the calendar we use, the traditional calendars used in the Hindu world, the Jewish world, the Islamic world, the Mayan calendar and I am sure there are more.
   
   
3. The theme could be something like "Mathematics is Everywhere" and develop the theme with a math trail. I did a search on Google for math trail and received a substantial list of links. There is a National Math Trail in both Canada and the USA. Many of the trails have outside activities but some education students here made a math trail that stayed inside the university buildings.
   
   

Let us know what you finnay decide to do and how it goes.