The Spanish version of Marshmallows are called 'Nubes' they are long, usually pink on the outside and white in the center- same concept, just a different look- but they don't taste quite the same. Nubes tend to be sweeter.
I have purchased American Kraft Marshmallows at Taste of America- so my friends there could try them. Toasted on beach campfires- they've given me mixed results- some like them, some don't.
As for Peanut butter- you can find it now in Spain- but its not a normal or commonly used spread. Most people know what it is, although not all have tried.
We have a friend that goes nuts for Pork and Beans- those they don't have there, we used to bring him a can from time to time
Nowadays though, especially knowing where to look, you can get almost anything over there... although I must say the Oreos don't taste the same (you can even tell they're different! Put an American Oreo next to one you buy in Spain- the cookie part tells all!
We used to take Nestle chocolate chips to make CCCookies- that was always a big hit!
Nothing like homemade!