I find that most people in my life have at least one "thing" they are collecting. It is often an insight into their personality and it is fun to hear people talk about something they are passionate about.
I have a 15,000 card Magic: The Gathering collection. Some years I’m into it, some years it just sits in boxes. I used to make some money trading them but it’s more effort than my usual work. It’s nice to get them out sometimes and go through them, lots of good memories.