Ride your bike instead of driving whenever possible--commuting to work or school, running errands, etc. It often doesn't take much more time than driving, and it's a good way to squeeze in more exercise. Bonus: saves money, good for the earth!
It's hard for me to find "extra" time to exercise. Setting myself up to bike whenever possible instead of driving has helped me squeeze in more exercise. I've found that often, it takes me the same amount of time to bike as it does to drive (or only 5 minutes more), too.