If you're walking off-lead at the park and see an on-lead dog approaching, it's good manners to put your dog back on lead until the other dog passes. There are a whole load of reasons why a dog may be on lead, not just that it's unfriendly with other dogs, and it shows respect for other dog walkers. If an owner looks like they're making an effort to avoid you (perhaps crossing the road, or going in a different direction) please do not take offence :) There is a thing called the Yellow Dog UK - some dogs need space and will wear a bright yellow lead to signal this.
If another dog has a ball, toy or stick at the park, be sure that your dog doesn't attempt to pinch it. I personally don't use balls at the park as it just causes a whole host of problems (other dogs stealing them and running off with them, resource guarding) but you do have to be aware of other walkers that might use them. Some dogs can get very protective over their possessions, it's difficult to tell which ones resource guard and which do not, so it's wiser to just try and avoid them.
Be sure to teach your puppy that it's not okay to jump up at people, and that it will only get attention when all four paws are on the floor. Unfortunately there are a lot of well-meaning but counterproductive people at parks that want to give your dog fuss even while it is jumping all over them and this just reinforces your dog to jump up. It's also good manners not to give treats to other peoples' dogs, at least not without asking the owner if it's okay first. But generally it's not a good idea as it can encourage dogs to go begging to strangers and may damage that dog's recall.
And of course the obvious one - always clean up after your dog's poo! And while it's not required by law, if your dog is sick, it would also be nice if the owners cleared that up too (coming from someone whose dog likes to eat vomit 🤢)