Tourist card
Here is a picture of what comes with your package when leaving Canada. If you do not have one you will purchase for $10 US just after you walk into the airport. Right in the middle of the arrivals area before you get to customs.
You will also be able to pick up the second form below here if you did not get them on the airplane or in your package. 2 of these (2nd picture) are needed.
After having your identification documents and/or passports checked and leaving the (1)bottom form with customs as you head towards the luggage carousel they will take both portions of the top picture Entrada and Salida form from you, leaving you with 1 of the bottom picture forms.
Now you are left with the above, after you check your luggage and pay $20 US departure tax they collect these at Immigration just prior to your carryon and yourself being scanned by security. If you do not have one they will give you one to fill out.