Honestly, Really nothing, perhaps some things when I was a kid but can't remember what those might have been.
Now I only collect supplies and books for the kinds of things I like to do, and there's quite a "collection".
When I was in high school my collection are Yugi oh! cards. In college I collects anime DVD's. Now I have no certain things to collect but my art utensils, Such as pen, color, paint, ink etc.
before when i was teen i just love collecting some anime stuffs like magazine, arts, figures, cards, etc. but now i love collecting dresses, sandals, and also anime stuffs.
Coins and Stamps. My brother and I used to collect both as kids and it was a something we never out grew. Its a bit more expensive now, but I still collect them when I can.