Barney used time travel as a pick up line and dresses up as old Barney to warn a girl she needs to sleep with present day Barney. So amazing, so Barney.
Robin: I'm moving to Japan. It took me a half hour to pack. Lily: A half hour? Robin: Well, it would have taken me 20 minutes, but a friend called me in tears
Barney [dressed in old man costume]: In a few minutes, the young me from your time is going to come through that door. Now, Cindy, I know this sounds insane, but in order to save the planet, you need to sleep with him tonight. Cindy: What? I... Barney: Sleep with Barney Stinson tonight, in whatever way he wants it, or he won't be able to find the solution to global warming that saves the human race