As everyone else says, "Be yourself". The Tesco carrier bag is a good idea, although I (a native) always take my camera bag when out for a day!

Above all - leave the headgear (Glengarries, Balmorals, etc.) at home! Americans have a passion for them - I think you feel 'undressed' without one! - but you'll really only see them being worn in Scotland by a few officials at Highland Gatherings, and by members of pipe bands (when they are playing!!).

You'll have a fantastic time in Scotland, I know it.