When I teach my photography classes, I spend 2 hours on basic accessories. Lenses, tripods, reflectors, flashes etc. Often people ask about the small doodads that you can get cheap, like adapters for the on-camera flash. My experience is those are fairly worthless and exist to part amateurs from money. That said, I can think of two oddball items I bought years ago that I wouldn’t want to be without.
One is a gel camera strap.
The other is a set of Opteka extension tubes. These run around $70 on Amazon.com, and are an alternative to macro lenses that run around $500. Anyone with a sense of adventure and an interest in the small would be well served by getting these.