(Disclaimer: Fairfield World and RJR Fabrics sent me some of the supplies to make these projects and Fairfield World compensated me for this post. All opinions, ideas, and enthusiasm are 100% my own.)
You can go for the Pi Puns and grab your favorite pie filling in fabric form. We had fun with this Cherry Pie. (White fabric is Encyclopedia Galactica by Andover Fabrics.)
Or you can go for completely math-themed fabrics. We loved the Geekery line by Sue Marsh for RJR Fabrics. This uses Scientifically Speaking in White and Black as well as Graph paper in green.
Or you can go for classic black and white. (Check out my favorite Coconut Cream Pie recipe here.)
You can find my complete step-by-step pattern and tutorial for making these Pi-Hot Pads on the Fairfield World site here.
You might also like my recent MATH paper-pieced quilt here.
If you're looking for additional inspiration, you can see my full collection of Pi-inspired projects here.