The easiest and most unique thing to write is a writing guide.
No other How-To guide can be so full of regrets and failed expectations, and still be taken seriously.
An unpublished writer can tell you what you should do, tell you the best way to get something published, how difficult that is, and the common traps to avoid.
An unemployed bricklayer will tell you exactly what to do to get all the bricks in the right place, and will avoid cliches such as “Don’t let yourself judge the wall until it is finished, silence your inner wall-critic.” in favor of “Don’t drop the bricks on your feet.”
Also the act of writing about writing is… well, writing. No other task or profession has a similar loophole or get-out clause. Talking about baking is not baking. Writing about train driving is not driving a train. Looking at flowers arranged by others is not flower arranging.
Most writing guides like to stress how important it is to just get down and write, no matter what you write, and something of quality will eventually reveal itself. Bricklayers don’t suggest that you lob masonry around until something that looks a bit like a house starts to form.