When I buy groceries, I do two brisk laps pushing my cart around the perimeter of the store before I shop. Then, I move as swiftly as possible down the aisles. Once the cart is full, I again do two quick laps around the store before checking out. This works in any big store.
For those who are at home or on the weekends, I recommend wearing an Ipod type device as you go about your normal routine. You'll be surprised at how often you'll just start dancing to a song you hear. This is a good way to add some unplanned exercise to your day.