If you cross the land border in your own car/on foot, you can use your passport and your CoPR.
However, if you use a commercial carrier (airline, or just a bus crossing the border) you need either a PR card or a Travel Document (a complicated process you cannot start until you left Canada, so not useful if you only leave for some days).
If you are from a Visa-exempt country, the airline or bus agency might overlook things and let you board because of your passport. Once you are at the border, the CoPR is usually enough. It depends on the company though if they let you board. I read in this forum that most Europeans don't seem to have a problem boarding flights to Canada, but that is just hearsay.