When this happens to me (and I live in Minnesota so outdoor activities aren't always an option) I usually put in a video I haven't done for years. It gets me moving, and although it isn't as intense as my usual workouts, I get pumped because I am always amazed that I still remember the choreography. This makes me want to keep working out the next day, and so on. But if the sight of your VCR or DVD player has you cringing, then take a walk, put on some rollerblades, give your bike a tune up, etc. Or maybe you need a break from cardio for awhile before you get right back at it. Have fun!