Okay, so you may have seen the title and thought "This isn't for me, I don't have a pet." Hopefully, you stuck with me to this point - Petiquette isn't just for animals and their humans. I'll admit, I almost typed dogs, but I have seen a lot of critters being walked in a park, so I edited.
Biggest note - Don't assume a pet is friendly. Always address the owner before you address the pet. For example - "That is a beautiful bird, is he friendly" before reaching out toward said bird.
Along those same lines, retractable leashes are the preference of some dog owners. I get that. However, allowing your dog to cross the street toward another person and animal while asking "Is your dog friendly?" is not a good thing to do. It has happened to me and I have been tempted to say no. What if there is no dog accompanying the other individual? Maybe that person is just afraid of dogs and you are causing anxiety. I know, hard to believe, but not everyone is a dog (or animal) lover.
Now that we have addressed passing, one other thought: Passing should always be done large human to large human. This way, the dog/dog or dog/small human don't have the chance to interact without the *ahem* responsible party verifying that everyone is okay with it. Kids get that - they even are usually the ones who have asked for permission before petting.
So, if we are passing large human to large human, that means that walking in parks is best done as it is on the road - on the left side (i.e., into traffic). This allows the largest *target* to be on the outside and address oncoming traffic (car, truck, bus, large human).
Retractable or not, except in specified dog park areas, leashes are the law. Attaching a leash to your pet's collar and then letting the pet drag it DOES NOT COUNT. Recently, during a walk on state game lands, we passed an area where someone had field dressed a deer the day before. Abby caught the scent and wanted to explore. Since leash was in hand, that did not happen. Not long after, we passed another pet owner who did not have leash in hand. Not wanting the dog to get sick, we warned the owner of the upcoming entrails. That owner, otherwise, might have had an interesting day.
Finally, poop happens. Clean it up. Yes, you can take your pet to PetSmart. Yes, they have islands of grass outside. It is not your yard. Other pet owners don't want to have to step over or prevent their pet from eating poop. Most of these locations offer poop bags, but you can get a bag container to attach to your leash or just stick a bag in your pocket. I am guilty of being caught bag less. I went into the store and asked for a plastic shopping bag. Worked just as well.