I bought a pair of shorts that weren't particularly exciting, but they turned out to be the most useful thing to wear for all kinds of unplanned activities: camping, gardening, hiking. Now I wouldn't dream of getting rid of them.

All the time. You buy a dress. Ten years pass. It's still great.
It was a Diane von Furstenberg sack dress in scarlet, navy and white. I wear it belted. You can shove that thing on and be ready for anything.
Women, tunics and sack dresses are your friend.

I’ve made a few purchases at thrift stores that were spontaneous and inexpensive that ended up being staples in my wardrobe, like a pair red Mexx jeans I found at a thrift store for $8 that I wear at least once or twice a week.

Probably that Fairisle knit cardigan!

Cotton stretch bras from American Apparel—but they bag out quickly, and now their sizing runs too big, so I can't buy replacements. Those were a great staple.

yes, an ivoire white leg pant

that happens often. There was a tunic dress once which seemed unremarkable for years until I suddenly had the right shoes for it.