Mexico City is all about the flavor. You can spend your days wandering through Roma and Condesa for art galleries, then hit the street stalls for the best tacos of your life.
Late summer is a smart time to go because flights usually dip. It's a bit humid, but you'll save enough to eat incredibly well once you land.
The intro window is a deadline, not free money. If you still have a balance when the 18 months are up, you'll start paying interest on what's left.