I would like to make some invisible ink that apears by itself after some time, preferably at least a couple of days. I want to write a dedication in a book with it, so it would probably be fine if the colour apears after half an hour of air exposure. Is this even possible using easy to get ingredients?
You might experiment with fresh-ground apples, which contain polyphenol oxidase and catechol oxidase, enzymes that turn brown in air (a tiny amount of copper or ferrous salt is also present, acting as a catalyst).
For that matter, dilute ferrous salts are often almost colorless (pale green) but darken to brown on exposure to air. (Digging in a podzol, soil of a pine forest, shows that color change.) So, you could extract a bit of liquid from greenish podzool and load your pen or marker with that.