Here's an easy Sunday morning DIY that literally takes 5 minutes.
I had a yard of vintage upholstery canvas with a pretty, 50's floral print on it.
Heavy upholstery and canvas fabrics work best, especially ones with prints, because they're going to be less likely to fray. However, the fray could be cute, so use whatever you have!
I simply traced a 3 1/2" square onto the back of the fabric in pencil, then cut out the squares. Instant coasters!
(Have more fabric? This would make cute matching placemats, too!)