The reason you instinctively feel that a walk in the woods is better than TV may be partially related to the schemas you've developed around both activities. You get the feeling that talking a walk is a more "intellectual" activity than TV because that's you expectation based on what you've heard, and your experiences of TV being a lazy activity for pleasure, not because TV inherently "rots your brain".
That said, the other half of things is probably the answer you're looking for. Right after class, your memories of the lecture are still malleable and not yet incorporated into long-term memory. The Wikipedia article on this is a good place to start: