I have to agree with rgf (is there a smiley for "waving white flag"???).
I think itīs more than enough to bring your host one little gift. Spaniards wouldnīt really understand the gesture of leaving a candy bar or some chocolate on your pillow every morning.
And one thing I definitely would not bring is Reesseīs Peanut Butter cups. I mean peanut butter is a little better known in Spain now than 10 years ago, but itīs not really THE thing to eat
What I think is very american is a Cheeseburger, although I donīt think the housekeeping service would think itīs very funny, finding a burger on the pillow every morning
Saludos
PS: To avoid missunderstandings-the last sentence is a joke, OK?