Cryptograms are great for these sorts of things - if you used a letter number cipher (depending on how good at these your guests are, you could just use a simple a=1, b=2 etc) and spell out a word that switches to numbers that are the combination, this could work with most lock types. If you want to get fancy, I just ordered this on amazon for an escape room-type event I'm doing for a client in the fall. They have some really unique locks for super cheap (I bought about 5 other ones as well and can't wait to start playing with them for inspiration).
I think that the storyline will determine your codes to crack - so they don't seem super random, that your guests find the codes follow a theme. Once you determine that, then it will all probably fall into place!