Always exciting to visit your potential future pup! 5-6 weeks old is a great time as they are starting to show some personality at this point.
Personally I wouldn't take anything on this visit. Just go and interact with the pups and have a good time. Make a list of questions you may have to make sure you don't forget anything.
One thing I would ask is whether crate training will be started before the pick up day. It's not imperative, but it's a good thing to know for day 1 at the new home. When you do pick up take a few old towels with you and rub them all over the other pups and the mom. This way you'll have something with a familiar scent for the pup to snuggle with.
Can't wait to hear about your visit!