I have a few more to add now that I've slept on it:
--play video games. This is actually quite cheap because I've been playing the same Wii games for over four years. Not bored yet. However, I might get back into World of Warcraft, which would mean a monthly subscription. Hmm...
--watch TV shows on my computer. I've been slowly making my way through all the classic Doctor Who. I'm a pirate >:] The Walking Dead, Game of Thrones (!!!!last night!!!!), Once Upon a Time, Modern Family, New Girl, Glee, and Chef Ramsay cooking competitions are among my favorites.
--clean. It's weird, but I really enjoy cleaning. It saves me money because I'll never pay someone else to do it, and my things are well-maintained because I care for them.
--masturbate and/or sex. ...what? Just don't get pregnant and this one doesn't cost much at all.