I’ve got a bit of a thing for note/pocket books.
My favourite by a mile are Rollbahn (esp the mini 112x138mm type)
Field Notes are also really nice (quite hard to get hold of in the UK though).
I find a handy pocketbook is much more useful than phone apps etc.
I carry two around in my work bag; one effectively a glorified ‘to do’ (or more accurately ‘don’t forget’) list, and the other for sketches, notes, ideas etc. (the fun stuff
)
I like ring-bound as they are easier to handle, and the ring binder bit makes a very handy pen holder!