You should try a rhythm game called osu!. It's completely free and it's a huge timesink. I even got my girlfriend to play it, and she doesn't usually play games on PC. Mind you, it has TONS of Japanese, and in general asian, music, so there's that. For the most part it's not the usual J-pop stuff though. Instead there's lots of music from the Touhou series, and then there's some more music from it. There's some western music too but not as much. Anyway, I don't have any experience with Touhou games but I think those are free too and should be pretty big timesinks. Also, some osu! gameplay here. I failed embedding. There's background and skins and stuff if you want but I think that video gives a pretty good impression about what it's about. Basically you click circles that appear around the screen, and you do that according to the rhythm.
Another recommendation would be Minecraft, for obvious reasons. I don't think it needs introducing