I think you're over thinking the whole "what should i wear thing" -- this isn't an interview; go dressed how you would normally dress. If your normal attire is sweatpants and a hoodie, then you might want to upgrade to Jeans and a nice shirt, but don't go in wearing a shirt and tie with khakis.
Making a first impression rarely has to do with clothing - it's more about how you come across in conversation. You'll get one, maybe 2 minutes max time with the manager. Make it count. Be transparent about the fact you've applied, let him know how much you'd love the opportunity to interview, and just mention you're there checking out a new product or something and just had to go over and introduce yourself. Let him steer the conversation from there.
Good luck.