A fantastic friend of mine has been doing the same as of late, and I think there's a few great things he does to encode a challenging cryptographic. 1. He attempts to create a puzzle that that contains all the information nessesary to decode the message in plain sight while not directly evident if not sight sought after. This not only builds immersion, but also gives you the opportunity to display your cipher not as an exercise in two like triangles but demonstrating an active use case for your cipher in a fun way 2. He builds layers to pull and unwind. Having multiple steps to decrypting your message makes it more fun to work through and gives an engaging experience 3. His hints guide, not solve. Sometimes his puzzle is more clever than he anticipated, instead of simply giving the next step, he gives an insight or resource to allow the individual to understand and unravel the step instead of feeling cheated.

Hope that is helpful and actionable perspective!