This is a short bit of game inspiration: Locked letters.
I had no idea these existed until I saw this article:
Sealed Renaissance Letter Virtually Unfolded
Meanwhile, Matt Riggsby could probably toss off 2000 words on the subject for a GURPS article in, oh, a day.
The idea of a letter designed to be difficult to open shouldn't surprise me but it did. It's a useful thing for games - especially fantasy or low-tech games, where "We just X-ray it and then let it pass on, the contents read" isn't an option.
Game rules? I can think of a few. A straight-up contest of IQ vs. the one who locked the letter. The intended recipient may get a bonus from a clue in a seperate letter, or a pre-agreed sign, or of some sort. If a lock-er has a signature style, and you know for certain the lock-er and that style, this may be worth a bonus of +1 (for a complex or wide variety of puzzle styles or mild clue) up to a +5 (for a completely consistent style or a blatant clue.)
You can also make it a contest of other skills - Mathematics (Cryptology), for one, or Cryptography for another - with appropriate TL penalties either direction! Skill in Hobby Skill (Origami) should work against almost any lock if I'm understanding them correctly.
You could, of course, just cut it apart - but then you can't re-seal it. And some fantasy materials may resist cutting, or magical reading . . . or contain traps (magical and mundane) that make circumventing the puzzle a very bad idea.