That's very kind of you, but actually I'm doing something else on Saturday.
谢谢你的好意，但是周六我确实还有别的事要做。
Well, I'd love to, but I'm already going out to the cinema.
虽然我很想去，但是我已经想好去看电影了。
I'm really sorry, but I've got something else on.
真是非常抱歉，但我已经另有安排了。
I really don't think I can – I'm supposed to be doing something else.
我想是没法去了，我应该还有别的事要做。