How about a . . . key?
In my recent creative writing class, I had my young writers “pimp” some keys by adding some ornamentation to them and then creating key tags that could go with them.
The idea is that a character in a story finds the key and the cryptic message and then needs to work out the mystery of what the key opens. In some cases, students have written riddles, while others have just written the name of what the key might open (for example “Monster World”).
Here are some of their final key designs . . .