Makeup, specifically eyeshadow palettes and lipstick/gloss. I've had a weakness for at least 30 years.
I also have a weakness for Hello Kitty. I buy little HK tchotchkes like iphone cases, key rings, and little figurines. I don't think I'd ever get anything obnoxious like a purse or outfit. It's mainly stuff that is inexpensive, unobtrusive and doesn't scream "UNPROFESSIONAL WEIRDO" at work.