A Montreal must, the famed Montreal bagel - like a NY bagel, but slightly smaller, denser and sweeter. The traditional flavors are poppy and sesame, but the rosemary salt is unique and delicious.
If you'd like to sit down I'd recommend going early because the line stretches out he door. However, you can also pop to the back and grab bagels to go fairly quickly.
Cash only so bring your Canadian dollars or you'll be slightly ripped off on the USD exchange rate.