I'll be quite honest with you - aside from the training problems associated with rushing off-leash work before they are ready. I would never let either of my dogs off leash in an area that wasn't completely fenced in. Even the most well-behaved dog can had a moment of distraction and get hurt.

I definitely agree with Shelteez2 - rather than going off leash to the park, why not buy some really long leads and work off leash with them attached. I used to put two 100 ft leads together and them hook the end to my foot. Leo didn't realize they were there, but I never had to worry about him running off. If they are loose, the dog won't realize they are there - but if he doesn't respond, you can reel him in.