A permanent marker is always good to have in the bag so you can quickly and easily write your name and phone number on your discs. It is good practice that when you find a disc on the course that is not your own, you send a text to the owner or hand it in to the course lost-and-found. That way the disc safely makes its way back to the owner, and you also more easily get your own lost discs back.
A Sharpie permanent marker is a strong marker that lasts a long time on your discs.