"Well, talking to you has been... extremely engaging, but I'm afraid I need to go..."

[Ok now pick an ending]

[A:] (either very lewdly or very bashfully)

"... take care of something."

(fuck me eyes)

[B:] "... return some video tapes."

[C:] ( do a bit of pirouette as if leaving abruptly, but actually complete the pirouette, ending with you much closer to their face, or maybe ear ):

"... ask you if you're busy this evening.

[D:] (begin singing 'Somebody to Love', but with the voice of NecoArc)

If the both of you have been mutually flirting for some time now, some part of your brain should have an inkling as to which of these is more appropriate.