There are lots of foods that better your mood. Ice cream, cake....
But they only better the mood until the sugar wears off and then the mood's worse cause now you're bummed that you binged as well as whatever was getting you down previously.
The walk is the best idea I can think of. Or even a run. Get your heart pumping. It works like the sugar, but doesn't leave you the additional depression over having done something you shouldn't have. It should in fact boost your mood more because you'll be left with the sense of satisfaction at having done something worthwhile to lift your spirits, of having accomplished something.
