What do I have to do to obtain a Canadian pilot's license if I already have an FAA one?
Transport Canada has all the information here for each FAA license type (and type ratings); the necessary forms and contacts are here. These are the basic requirements for a private pilot, for example:
6.1 Conditions for Conversion - Private Pilot – Aeroplane.
(1) Must hold an FAA Private airman certificate with airplane category and class rating(s).
(1) Age: Must be at least 17 years of age.
(2) Medical fitness: Must hold either a Category 1 or Category 3 TCCA medical.
(3) Knowledge: Must pass the Conversion - Private Pilot Licence – Aeroplane, (FAAPA) written examination on air laws and communications. Must be able to read, write and communicate in English or French.
(4) Experience: Must provide proof of a minimum of 45 hours pilot flight time.
(5) Skill: No additional practical flight test is required.
If anyone came here like me looking for the ability to just rent in Canada once in a while, You can go here and fill out the form. Apparently it only takes 2 days and costs $45 to get the Foreign License Validation Certificate.
You have to actually go to a Transport Canada office, which is a huge inconvenience if you aren't in one of the few Canadian cities that has one, but when you do go it's pretty straight forward, and your license validation lasts for a year.