Just like Iris said posting WIPs of your comic pages, bonus art and promotional art is some good stuff to put up onto your ko-fi!
Using my own page as an example, I've got gallery folders for WIPs and process gifs/timelapses, Comic art and comic previews, commissions and requests and for art challenges like drawtober of huevember or mermay
Posting things like WIPs is especially easy since it doesn't require taking time to make additional content and thus letting you focus on working on a page plus is nice incentive to readers who support you