I think a good thing here would be an FAQ, or some background information on how you built the pages, old character designs, rough sketches, etc. Stuff that you may have laying around behind the scenes that you don't normally show readers. This shouldn't require too much extra work on your part (I think).
If that isn't what you were thinking of, maybe do an art request for something your fans might want to see. Or maybe some cameos if your comic allows for those. Another option is letting your readers decide on an interaction between your characters that occurs in the comic (obviously not one that is too plot sensitive, however. Don't want to accidentally change the story!) These are more labor intensive though.
