I've been learning Spanish with free lesson "tapes" from Language Transfer (which I highly recommend.) The lessons are a collection of short audio MP3s and take the format of conversations between a teacher and a student; he introduces the language a bit at a time, and asks her questions; the listener is supposed to pause the track and answer each question, then hit play and hear her answer (which is sometimes wrong, of course.) It's a great format, but the only time I listen to the tapes is when I am walking somewhere. And this has produced an interesting effect.
Perhaps you've heard of the method of loci for memorizing a list of items. You imagine a walk through a familiar space (perhaps your house or garden), and associate each item on your list with a location along your journey. (This is also known as a "memory palace".) It's useful because people tend to have better spatial memory than arbitrary list memory: When you go to recall the list, you imagine that walk through your house and the items pop into your mind as you "encounter" them. Spatial locations prompt memories of the list.
In my case, I'm taking various walks through the city. I've mostly been listening to the tapes straight through, and so it took me a while to notice what was happening. But then I re-listened to a tape to refresh my memory on an older concept—and I remembered where I was and what I was doing when I listened the first time! The student is having trouble with the "soft B" sound of "V" in Spanish? Oh yes, I was looking at garlic in the grocery store. The teacher is going over how verbs take the -ado/-ido suffix after any form of haber? Yes, I was walking back from Porter Square, 5 blocks from home. Items from the tapes were prompting memories of spatial locations!
This is, so far as I have been able to tell, completely useless. But it was a fun surprise.