When I was younger, I used to collect Pokemon cards, stickers, fake tattoos, No. 2 wooden pencils, erasers, and rocks.
Pokemon cards are collected by many many people, and I would do the same. I loved the different kinds of artwork provided for each Pokemon and thoroughly enjoyed looking at them. I had dozens of sticker books full of different kinds of stickers ranging from different colors and images. I had a load of no. 2 wooden pencils with different patterns on it as well. I used to keep them all in pouches or a huge pencil box. The erasers I collected were also of different kinds: erasor tops, small, big, round, diff. colors, with images, or none. My favorite kind of erasers would have to be the Pokemon eraser tops, like these:
As for the rocks, I had gone to a rock museum for a few times by school field trip or going together with family. I would collect rocks or stones that I personally thought looked nice.
Lastly, the fake tattoos I would collect were ones you could get from vending machines, like these for example:
I had collected a hefty amount of them during my childhood but unfortunately I lost most of my collections while moving into a different home. I have a few pieces of each here and there but I would not constitute that as a "collection" anymore. Maybe I'll try gathering some up again some time