One of the best items a GM has ever given me was a series of 3 spell scrolls with homebrew spells on them, each of which was encrypted using a different cypher. At the time, but the GM and I were uni students doing a course on security, so it was fitting. One had a riddle, where the answer to the riddle was the key to a Vignere cypher. A second was a series of lines that formed a hexagonal variant of a pigpen cypher (but I forget the precise details).

The third we never actually managed to solve fully. At first it appeared as a bunch of letters in a grid. We learnt that it had a map drawn on it in ink that revealed itself under the letters when the scroll (a physical prop the GM gave me) was heated. A riddle given alongside the puzzle was obviously meant to be used with a transposition cypher on the scrambled letters, but we never worked out exactly how to transpose the letters.

Riddles are a classic part of fantasy RPGs, but far too often they're incredibly dissatisfying or even frustrating because they both require out-of-character solutions and they block your progress in the story until they can be solved. I really loved these scrolls because it tied the solution to the puzzle to something completely separate from the main quest. It still required solving out-of-character, but at least I could take it home with me and work on it in my own time, rather than being distracted at the table with it.

edit: oh :( you deleted your comment while I was writing my reply. I enjoyed your reply. It was a good contribution to the conversation.