Sometimes, when I get bored of doing quests(especially the long ones), I just go into a killing spree. And my favorite enemy? The Giants. Just get out of Whiterun, wait a few days, and go to every giant camp that you see on your compass. It may be not THAT fun, but it's good to pass your time.
Also, putting the game's difficulty on Master, and even downloading some mods to make the game harder, to make you feel like you're in hell, can help you pass a random night.
The best alternative, of course, is to download ''Mission Mods''. Some of them are extremely hard, and you'll probably take hours to beat them, and some modders are creative and respectful.
Such a shame that I got banned from Nexus /: