I guess it depends what level you're at and how quickly you can pick things up.
If you can walk/run without help (like a wall) then I suggest a quiet path with paved/tarmac footpaths or a large carpark on a quiet day (just be alert for cars reversing etc and try to find an area with no cars at all).
Failing that, promenades are really good, civic centres usually have lots of space and concrete.