I dick around editing skate footage I've shot of me and my friends. I've made dozens of edits out of boredom that will never see the light of day haha. See if a song fits a certain part, see if a clip looks cool in slow-motion versus regular motion, etc.
I also make remixes of pros' footage occasionally when I have large blocks of time on my hands.
https://vimeo.com/user12355909/videos if you're interested. Some of them have garnered a lot of attention, which is pretty cool.
Reading is amazing, and super cheap if you do it right. Get a library card ASAP! My local library has a searchable online database where you can place holds on books, CDs, movies, etc. You can place holds on whatever you want, then have it delivered for pickup from the library closest to you. It's awesome.
Whenever I'm bored I'll just hop on there and put holds on a couple books and within a few days I have them in my hands. If they don't have a specific book I want I can just file a request and they'll usually buy it.
Your library might not be quite on that level, but at least check it out.
Besides that, the usual hanging out with friends, messing around on the internet, listening to music and the like.