I found a great list of questions from here, and this was one of them.
I'm pretty good at not mindlessly collecting, wanting to be able to live out of a box and be happy, preferring the form and function of things that have obvious, general, immediate, and non-redundant purpose. But, I still do it some.
Clothes are probably my most regrettable category of collected things. My weight yoyos a lot, so 20% of my life's time I have a stash of clothes that are too big, and 80% of the time I keep an aspirational stash of clothes that are too small. Then, I also find myself buying replacements for clothes that are nearly worn through but not quite, and I keep those near obsolescent clothes around just in case. Some items I keep wondering if they've only briefly gone out of fashion. Oh, and I'm the worst with jackets, buying them hoping they're The One, only to wear a damn hoodie regardless of the weather.
Old electronics are another category - maybe that old laptop has something on the harddrive I'll regret deleting.
Books are one I don't regret. I've gotten in the habit of buying anything that I'd like to read should I have the time to read. I usually discard things I've already read, giving them away to friends or donating them, but I don't mind keeping things I'd like to read. I view my bookshelf as an index of rabbit holes I might want to visit.
Lately I've also been semi-mindlessly collecting bitcoin swag, stickers, and posters, and repurposed bitcoin things.
What do you mindlessly collect?