What should I tell my girlfriend to bring her to the rooftop?

The easiest way is to tell her you’re going to a speakeasy rooftop bar. We’ll meet you in front of the building and play along. When you arrive, we’ll ask for the name of your “reservation,” and then guide you upstairs. From her perspective, it will feel like you’re being taken to a speakeasy rooftop bar, until the surprise moment.