I'm sure it's been brought up many times here, but the recent thread made me think of it. And he wrote enough stories that could be called science fiction to fill one of those Penguin books. These are the ones I can name off-hand -
ARTHUR GORDON PYM - his only novel.
HANS PHAAL - his longest short story (I guess).
THE MAN THAT WAS USED UP - a "bionic man" story.
LOSS OF BREATH - a "suspended animation" story (sort of).
SOME WORDS WITH A MUMMY
MELLONTA TAUTA - a futuristic comedy (although he used it for a lot of 19th Century topical jokes).
THE BALLOON HOAX - a SF story, since no one had crossed the Atlantic in a balloon yet.