If someone gets mad at your dog because they are jumping/wrestling with another dog who is enjoying it... well, then they are dumb. Lol. Dogs will be dogs -- it's how they play. Jackson makes a ton of loud noises while playing and wrestles, etc, I've never had anyone have an issue with it. If he's barking too obnoxiously, I will stop him, but if he is playfully barking at a dog, I let him. It is a dog park afterall.
Poop bags- I always bring them. Our local park has poop bags usually available and pooper scoopers. But I bring my own just in case. I always bring my own water too usually, just because sometimes the water there is dirty. Treats... I think it just depends on the park.
I belong to one dog park where we pay yearly for it (it's in a state park) and I find it's a lot more strict when it comes to rules. People are a stickler for rules there... and they are enforced. So no treats are allowed there and someone told me off once
I go to another dog park that is very laid back and sometimes I prefer it, the people are a lot more normal, LoL. But there's also not as many small dogs there. Anyways... I'm rambling, but I'd skip the treats.
Basically, just go to have fun!!! If for some reason your dog is uncomfortable, leave and try another time. But I can't stand when people pick up their dog for unnecessary reasons in the dog park.. it makes the other dogs curious and they start jumping on the persons legs who is holding the dog, etc. Also, let the leash off asap. When people leave the leash on the dog as they walk in, it often just makes things worse. When I get to the dog park, I unclip Jackson before we walk in, and usually we are greeted by a big group of dogs and you just kind of have to push your way through and continue walking. Don't stop and let all the dogs get out (they have double fences).