I second the Tripod series, which I read in elementary school (actually, I started with the prequel, "When the Tripods Came," and read the others a few years later).
2001 was another I read as a wee lad, though in middle school.
There's a few novellas by Heinlein that were fun to read when I was about that age: Tunnel in the Sky, Citizen of the Galaxy, Farmer in the Sky, Space Cadet, Between Planets, Starship Troopers, quite a few more. Mostly his earlier works.

Asimov and Clarke are also be worth looking at.