It's been about 10 years since we had a dog young enough to need (want) off leash exercise. We were almost always able to find an area to play fetch with a tennis ball, frisbee or stick. As was said, you have to have a iron clad recall and be extremely sensitive to your surroundings and be a "good citizen."
Even for those times where walks were the only physical exercise possible, we were able to satisfy the energy problem by providing mental exercises/problems that went a long way to treat the "got to do something" urge. I really like Kongs and regular obedience work.
Doug, DVM