I am planning on running a game (not sure what system yet) with pre-made characters for a few weeks while one of my friends is back home for Christmas.
I had an idea that one of the characters has blocked out a portion of his past, which he will remember once he visits a particular scene. Once this character remembers this, I would remind him about an item he once stuffed into a hidden pocket in his coat, which is still there now.
What I would like to do is use some kind of invisible ink to add the item in the hidden pocket to the characters inventory, and upon him discovering it have the player write over the invisible ink with something to make the item visible on the character sheet. That way it really was there all along!
Is there a way to achieve what I'm looking for? Can anybody recommend either a product or a home solution? A solution that works for multiple weeks would be preferable, but I can work with a solution that lasts for only a few hours if I need to.