Why you should give Obsidian's Canvas a try
No doubt you're using Obsidian already. Obsidian is my favourite editor for Markdown, as it's mature, stable, extremely well-maintained and I don't think it's going to be enshittified anytime soon. Most importantly, the files in an Obsidian vault can be used in any text/Mardown editor, so the data is fully portable.
If we want an Obsidian vault to be more analogous to a notebook, or a scrapbook, we'd want the ability to add simple diagrams and post-it notes, or snippets of text and images.
We can do this using Obsidian's Canvas feature. If we select 'New canvas' instead of 'New note', we get a view in the main window to which we can add 'cards' that can contain text, images and code. Cards can also be placed in group containers. Obsidian can even convert any of the 'cards' to a Markdown note file, or import an existing file into a card.
Are canvas files as portable as the standard Markdown notes? Sort of. If we open a .canvas file in soething like Notepad++, we can see the canvas data is stored as a very readable JSON array. The cards' contents can be copied and pasted elsewhere, or anyone could write software, incorporating a JSON parser, to use the .canvas file as a data source.
So, if you're into using Obsidian as a notebook, I definitely recommend giving the canvas feature a try.