Lettering, numbers, pictures that are part of a cookie or cake itself. (It's all one dough).
more examples of "מבטל כלי מהיכנו" (nullifying a vessel from its normal usage) that come up on Shabbos;
understanding מקלקל, working with an attic door;
what do we call "קיימא" (permanence) when dealing with stickers on clothing and other items;