I have also tried different formats of notebook, and for a year and a half I have a phone from which to send things. Tried to install a note-writing app and failed thrice, now I only send myself emails.

I send photos of locations. It has already come in handy when I had to remember the way to a population. It felt like magic - I really could see the same exact tree, not rely on the general sense of location. I think I will use this to train my visual memory with other places, in the city, although I do not travel a lot and so this might become boring soon.

Also, photos of fields of view through my microscope. That time I had no choice of camera, and it took a lot of swearing to actually capture the images, but a digital archive is still a good thing. Perhaps I shall make a separate gmail account for photos only.

Photos of timetables, crude maps, parts of plants (definitely beats carrying a herbarium press, although not as reliable since you might miss the crucial feature; but bio ethically, a win), of my kid (I am guilty of not writing letters, but a picture without words is still better than nothing), etc. 'A good naturalist must draw well' is a rule that always frustrated me, since I don't draw well. I once went to great lengths to draw a young plant in my notebook, more like map it in actual size. I am still proud of that picture. However, it turned out to be a species of Dictamnus, and my sister (who helped me) still has stripes on her forearms which don't tan well. (Kinda like scars, but without puckering tissue. We are twins. I don't have anything like that. Must be the whole 'my object of study' effect.) Were I to just take a photo, the lesson would be less painful.

As to notebooks, I carry a small one with lined paper and a pencil holder in my little bag, for ideas/addresses/stuff to buy/...), a middle-sized grid-paper one in my main bag, and several stationary at home, for recipes and the like.