Congrats on the upcoming puppy!!!
As to advice: Most importantly, don't stress about it! Just go there and enjoy every minute of your visit. Second most important is don't forget your camera and have fresh batteries. Thirdly (sorry, I just have to say this as obvious as it sounds), listen to what the breeder has to say. The responsible breeders want only the best for their puppies.
We have a rule that we think is a good one for people visiting puppies and that is that they have to sit on the floor while holding a puppy. Those little guys are amazingly squirmy and fast and you don't want to drop one. I know a lot of people maintain the test of holding a puppy on it's back is good, but my personal opinion is that this is not as important as some say it is. If a puppy won't do it at first, don't worry about it; just keep working on it and it will happen as the puppy gets to know and trust you. Certainly, you can bring a toy and a lot of our puppy people do. Just don't be disapointed if the puppy doesn't play with it at first. Every puppy likes different things.
When we have people visit their puppy, we usually keep the littermates in their x-pen so the people and the puppy can concentrate on each other and that's what I would suggest, at least initially. Later, if you want, you can put the puppy with it's littermates and watch them play. When it comes to puppies, there's probably nothing I enjoy more than watching puppies play.
Good luck this Sunday!