Garbage cans and doggie bags for cleaning up after dogs are essential for basic good health for both dogs and humans. Many canine diseases spread through feces, and feces attract insects, which can spread the virus to humans. Remember the old guy yelling at you to clean up after your dog; this is a demonstration of good citizenship we all should practice.
Parks with a two-gate system are something you should seek. The two gate system helps avoid the possibility of dogs escaping from the area; the second gate also increases the safety of all the dogs.
There should be a pleasant shady area where dogs can lay down, cool themselves, and rest before continuing their active play. Dogs must often drink to cool themselves; dogs can't cool themselves as efficiently as humans. Dogs play very strenuously in dog parks. Water is an absolute necessity; if there is no water available, dogs may suffer from severe heatstroke, which can be fatal.
Get some elbow room to avoid crowding. If dogs become too grouped up, it is much easier for a "bad dog" or a pack of evil dogs to corner and harass other dogs. Fights break out more often under overcrowded conditions.