I know that this is a stretch but donate them to a local high school, community college or university that has a sewing skills, fashion design or architecture department. I work at a university that has all three of those departments and I see cutting mats in all three areas. The students can always use more cutting surfaces in the classrooms. I found one mat in the trash last summer that was so old...."How old was it?" It was so old that... it had been used so much that the surface was not long flat but covered with ridges from the number of years of cutting, and cutting and cutting in the same areas.
I am sure donations of even the mat you don't like would be appreciated.