Maybe your grocery shopping didn't take as long as you thought it would. Maybe your boyfriend did your laundry, and washed your car. Maybe you just weren't that great at time managment today, and your schedule is resultingly off-kilter. Whatever the reason, you've suddenly got thirty mintues, and it's time to kick ass and make use of that gift.
Got 30 minutes? Try walking. Exercise is anything that makes you breathe heavier than usual, and every little bit counts, even if you are just walking. Plus, walking doesn't require heavy duty sports wear. It doesn't (usually) require a shower afterward, or a planned route or time frame. Just walk for fifteen minutes, turn around, and continue on with your day. You've done it, you've done something extra to help your health goals.