Truly anything that comes to mind. Quotes I like, ideas, thoughts on anything at all, to-do lists, etc. I've seen people draw in theirs, as well. It's really a free for all. I'll be real I tape sticky notes and keep stickers in mine lol
But speaking of walks! You should totally write about what you see on your walks, try to pull meaning even from the smallest things! Poems about ants, how you liked that one persons hat, anything at all is worth jotting down imo