Yoga would be great for your days off. It'll help stretch out the muscles you use running and increase your overall flexibility and strength. I personally like Ashtanga Yoga (it's a book) but If you don't already know how to do most of the moves I would suggest a yoga dvd, Gaiam has several good ones for beginners and hgher. I believe there's even some yoga for runners dvd's or books on amazon.(that's where I get all my dvd's). And trust me they are NOT just easy stretching, you will work up a sweat with most of them But it would be a great change of pace for you. I try to do yoga everyday along with other exercises but for the last month or so I'v had some medical complications that have made it hard to do much of anything.