Sci-Fi Books for Pre-Teens?

o2binbuzios writes: I have two pre-teen boys who are avid readers, and I am going through my mental register for GREAT Sci-Fi & Fantasy books for them. What are some of the classics (and maybe new additions to the classics) that would be great for them to read? I am asking because some of the 'straight-up' classics I remember actually seem kind of dark & cynical for younger readers. Starship Troopers and some of the other Heinlein are definitely darker and more political than I remember...Foundation Trilogy and psycho-history maybe too dry.. Road-trip reading season is upon us — what are the good reads for the kids in the back seat?