Field Notes or similar formatted notebooks is what I use when I need a very portable notebook. I usually use them in my Belroy cover. I have a trick where I connect them together with paper clips if I need more than one.
At the moment my only use for it is for capture on the go when I don't feel like using my phone. For not so long ago I kept a paper log in one and in another I did some habit tracking. So I varies and changes a lot.
Field Notes is not a great notebook from a paper perspective. And I wish I could find something locally available with a similar format with good paper. So far I have not.