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 catalog 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?"