Videogames. It has not been super expensive as I enjoy indie games the most, but still.
Pen and paper organization. This is recent. Due to a couple of mental disorders, I have problems remembering things and keeping organized. I was using a to-do list for my phone, but it was becoming less and less effective with time.
So I found a weekly planner online and I bought it telling myself that it was expensive, but it would be enough for a year and I wouldn't need anything else.
The planner has been great, by the way. Yet, when it arrived, I liked it so much that I had this classic feeling of not wanting to ruin it with my handwriting. I needed a good mechanical pencil! Erasable, yet stylized.
Then I thought the pages looked clean, but monotone. Stickers! What about my own creations? Thermal printer with sticker rolls! And so on and so on.
I am productive ...and addicted to stationery items.
Buy a good mechanical pencil and coloured and black leads made by Pentel, Ain Stein model, or Uni's equivalent. You need not sink money when you get the colour variety with leads.