Well, if you want stand-alone + sci-fi, try "the Dark Side of the Sun".
Otherwise, you can pick up any first book from one of the Discworld story arks. Personally, I'd go with "Wyrd Sisters" (witches story ark) and the City Watch story ark. Although the first book in that series is "Guards! Guards!", I'd recommend starting with the Second book, "Men at Arms", which is more representative of the rest of the series, and just more fun AFAIC.
Otherwise, you can pick up any first book from one of the Discworld story arks. Personally, I'd go with "Wyrd Sisters" (witches story ark) and the City Watch story ark. Although the first book in that series is "Guards! Guards!", I'd recommend starting with the Second book, "Men at Arms", which is more representative of the rest of the series, and just more fun AFAIC.