You could read the world's first Sci-Fi novel, Lucian of Samosata's Ἀληθῆ διηγήματα. - Edit 1
Before modification by Tim at 27/05/2012 03:12:28 PM
Edit: just saw someone else already recommended that. If you want to read something you already have a copy of, I'll second the recommendations of Aristophanes and Herodotus. I like both of them.