I like to keep anything that is slightly sentimental to me. I'd probably be a borderline hoarder if I didn't force myself to throw away the more mundane things that I have inklings to keep. Granted, sometimes I just keep things (such as receipts) because they get put somewhere and then I never think to throw them away. Still, I have a lot of things that most people probably would have thrown away by now.
In elementary school my friends and I liked to make up freeze tag games. We eventually had so many different types of freeze tags that we couldn't remember them all, so me being how I am, I decided that we needed to write down a list. I remember that I took that list to recess with me every day to make sure that we remembered all of the freeze tags when deciding what to play. (And I typically wanted to play Halloween Halls instead of freeze tag. We made up all our own games if you can't tell.) I still have that list. It's wrinkled more than it even should be because I had this fascination with how soft paper got if you wrinkled it up, so I used to purposefully wrinkle any paper that wasn't school work. I'll probably never throw that list away, even if it's pretty useless these days.
That's just one of the small, sentimental things sitting around in my room. I have the usual ticket stubs. I even had a piece of bark from the tree that used to sit on my elementary and middle school's playground. They cut it down the year after a graduated. None of these things serve any purpose other than bringing back fond memories whenever I see them, and that's why I like them so much. A lot of things have changed (especially since elementary school), but I can still look at those things and immediately remember a bunch of memories associated with those things, even if it's really only a list of freeze tags.