Outside Workouts

I like to ride my bike for exercise, occasionally (instead of transportation...) -- lately I've been parking at the airport and making the BWI loop.  But I'm interested in a little more -- anyone have any ideas? 

Walking, jogging, "running"  -- same thing.  Flats preferred -- I'm lazy that way.

I've heard the C&O canal is great for biking, running, walking, etc. and mostly (if not all) flat.  I haven't tried it yet though.