All of the above. Unless it is a remote location AND you have excellent recall, wait to find a fully fenced park (or 'natural fencing..water, etc.).
No treats or toys in the park, just invites issues with other dogs.
Off leash is best, a leashed dog may feel vulnerable and therefore be more defensive.
Make coming to you and going home a treat, keep something special in the car as a treat as soon as you get to the car. (A friend who had been a K9 Cop did this with his rescue basenji, took about 2 weeks before Charlie beat Tom to the gate. Tom used hot dogs or hamburger patties as special 'car treats')
Work on his recall, and when you finally do go to a park, keep your tone and attitude relaxed and all about fun. He will pick up on anxiety and be stressed.
We are dog park fans, but not just any park. Needs to be nice with nice people and dogs. Some are not so much, if those were our only options, we would not go. Visit by yourself at different times of day, meet some people, watch the dogs and interactions. You would 'interview' a daycare, do the same with a dog park.
Good luck, have fun!