I used to have those days also. I will highly recommend that you on those days try HARD to get to go outside. Take a little walk. Usually (at least for me) it's not really the walk that I don't want to do. It's the front door being in the way. As soon as I am out of the door, I have no problem doing what I'm supposed to be doing.
Think of all the extra calories you'll burn. Lying on the sofa is the one thing you can do that burns the least calories, unless you're actually sleeping. Even sitting up playing your game will burn more.