Starting January 23, US citizens will need a passport to return from Canada, Mexico, and the Caribbean.
If you don’t have one, you can apply at selected Post Offices. Lines are expected to be long given a last minute passport rush on top of the holiday season.
CIBT can get you a passport on an expedited basis. I used them in 2005 and their turnaround was 24 hours vs. 4-6 weeks from the Post Office. I overnighted them the forms and documentation, 24 hours later they overnighted my new passport back. I’ve no idea how they do it.