This particular topic by Long and Short Reviews was easy, as I had a fairly extensive collection as a child, sadly having long since been thrown away.
All my friends collected them, and we would take our collections round to each other's houses and swap them. I can distinctly remember one friend who had a lucky rabbit's foot she had got from a trip to Disneyland, which unsurprisingly she was never willing to swap out no matter what I offered her.
My personal favourite was a small paperweight containing underwater items such as a shell and a starfish, plus a little egg timer. I can't remember who I got that from but I valued that one with my life. There was also a skeleton with green jewel eyes which I think everyone had some variant of, and for some reason a hotdog that I refused to let anyone have.
These days I collect autographs. I'm a member of two autograph collectors' forums where we share addresses and successes from writing to celebrities, and I have a fairly vast collection now including two prized personalised photos from Alan Rickman and Gene Wilder. I even got a two-page handwritten letter from Tatiana Maslany just before "Orphan Black" became successful - fairly rare from any celebrity.
So what did you collect as a child?