Playing video games, browsing forums as well as running my own, reading, listening to music, etc. When I was younger, elementary to middle school age, I believe, I used to collect Pokemon cards. I don't anymore, but it wouldn't be a half bad idea to make an investment out of it, make some extra money. I've heard those things can sell pretty good if they're in decent enough condition.
I love to work out (That is why I'm a Personal Trainer as a career) I love to play sports, watch sports, hiking, Collecting sports cards, playing board games, video games, reading books, watching movies, posting on forums, etc.
I collect Fire/EMS/Police Department patches. I have a history of being an EMT myself so I love the designs of the various arm patches. Here is a sample of my collection. This is probably about 1/5 of the amount of patches that I have...
I also do a lot of offroading and automotive modifications. I'm with a lot of different Nissan Xterra groups online and sometimes we go out to find some mud!
Well, as a kid, collecting Pokemon cards for me wasn't really an investment, like it is for hard-core fans. I just got the ones that looked kind of interesting. I did have a binder full of them at one point though!