My choice is as simple as it is obvious--
Made of the most weather-resistant material I could find, a copy of The Complete Works of William Shakespeare (and maybe a quote or something from any of my works, if I have anything published by that time.)
Even if no one ever read it, it'd be nice to know Hamlet's words, forever true--"There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy"--would still be readable and existent even if we ruined the Earth...
Mozart, Beethoven and Bach already have recordings of some of their music on the Voyager Golden Record...they're dead and gone, but their music might last ages...it'd be nice to see some of the great words and thoughts of humanity be preserved that way, even if they'd be forever indecipherable, just to know they'd still exist (and then, of course, a garbage scribbling of my own...because if I'm spending all that time in a cramped capsule to make it to this place, damn it, I'm leaving something behind of my own too!) :)