Well I blew it and went on an overseas vacation, forgetting my PR card at home. What should I do?? I am a US citizen. Could I enter as a tourist and then come back to show them my PR card? Thank you!
Bad news: You cannot enter Canada as a tourist if you are a PR.
Good news:
If you hold a valid U.S. Passport you should* be fine. A valid PR Card is
only required by a commercial carrier (i.e. airline), so as long as you do not tell them that you are a PR of Canada, your U.S. passport will allow you to board the flight.
Once you arrive in Canada, just bypass the PIK machine at the airport and proceed to the CBSA offer to explain. The officer will just need to verify your PR status, which should be easy for them to do.
[*] some airline employees may not be aware that a U.S. citizen with a valid passport is the exception to the rule for a PR of Canada to present a valid PRC. If you run into trouble, ask to speak to a supervisor.