Spainards are more formal than people from the USA, for example, no nightclub will allow you in without real leather footwear (non-sneakers), slacks, no jeans, teeshirts, sport shirs, etc., and definitely NO athletic wear! Otherwise, we dress pretty much like people from the US (calling yourselves "Americans," by the way is very insensitive, and rather ignorant, you aren't the only "Americans" so it can be rather ambigous), and the brands, styles, etc. are (lamentably) those you will find back "home," due to the globalization and gobbling up by the (US)corporate machine. The same comment goes for many things in the culture, like (my G-d!) music, for example.

Be that as it may, it also means you can purchase a pair of Levis, for example, and wear them without unduly sticking out like a sore (blue) thumb. Sizes are different, being metric. Toothpaste, and other food items are well stocked in US brands in the supermercados, especially in Corté Ingles, the huge department store chain. The Spainards have had the good sense to retain their vino, chorizos, pan, tortialls, churros, chocolaté caliente, jamón and other necessities of life, so you won't be deprived of the good (Spanish) life.

Dig into it with both hands!
¡Saludos!