Travelers notebooks is this awesome covers with all kinds of cool accessories and stuff to fill them with. They come in two formats, one that is like a skinny A5 and one that is kind of like Field Notes.
I used to use them for a long time as my go to notebook for everything. I used to fill them with one lined notebook for writing, one dotted for managing tasks and such. I used to like it a lot. It was very compact and awesome.
As I discovered local stores in Norway that let me order Leuchtturm1917 notebooks, but not Travelers refills I slowly moved away from the system.
The standard one can fit up to three refills.
There are also tons of third party covers that can do more. Plus a lot of accessories to make it your own.
I'm seriously considering moving a bunch of my stuff back over to this system.