As I am in the middle of a move, this idea has reached my heart. It's about collecting the keys of your old houses (we know that you have not returned them, pillines) and place them in a box.
This picture It has its touch of emotion, especially if Under the key you write in what years you lived there or where it was located. And if you want to complicate a little more, you can paint each key with plastic paint, and paste them in a paspartout with a paper of good weight, so that it becomes more "professional".
I think it's a beautiful way to show your memories, although I must admit that the new keys are much uglier than the old ones. In fact, if you want to do the same with old keys, stop by an antique dealer or a rake because in many places they sell them. Surely you will look great.