Diary is not that useful right now. I plan to use it as a tool people can use when they forget something. Things like showing you the motorcycle controls and speed switching with speed potions. It will also have a main quest summary and track important events that happened and locations you visited and unlocked.
I could use it for tracking recipes and items, but I don't really wan't to go overboard with this since I want to strike a balance between giving enough information and keeping some things secret. I actualy want people to miss things on their first playthrough, so that the second one is different.
The unlocks screen kinda fulfills that role alreay. When you finish the game, the title screen changes and new "unlocks" option shows up. Here you can see all the things you unlocked while locked items are shown as "???"
I plan to add bestiary into the game at some point as well as weapon / armor collection. I will most likely put these in the main menu showing progress from all your saves.