"hi, can I buy you a drink?" is also bad for other reasons, because this often opens a kind of transactional model of things where there's kind of an idea that you're buying her time, either for conversation or for other more intimate activities later. Now, this isn't explicitly the case, but it can get really awkward, so I'd seriously caution against opening with it.

That sort of things vary a lot depending on what kind of culture you're in.