What works for me when I feel I'm in a rut is to play a game that doesn't take too long to complete. 10 hours or less usually does it. The reason I do this is because the story flows quickly and it gives me sense of accomplishment when I'm done with it.
You can check how long a game takes at http://www.howlongtobeat.com/
But be warned: Pick something that is actually worth playing. It won't help if you grab a game that takes 5 hours but it is absolutely crap.
Many games get shot down in the reviews because they are too short. These are the rut-killers you are after.
