You shouldn't have any trouble getting back into Canada since you're traveling on a US passport and are visa exempt. My husband left Canada for two short trips to the US after landing and before the PR card arrived and had no issues re-entering.
Good question... Couldn't remember... Had to ask...
He traveled by bus from Toronto to Buffalo (to catch a cheap flight from Buffalo airport).
He had both his passport and also a Nexus pass with him. He did mention that he was a newly landed Canadian who was still waiting for his PR card to arrive. The border guards didn't seem fussed at all and just waved him through.
From what I understand you should be ok -- it might not hurt to have your COPR with you -- I've read that's what they ask for if you try to apply for your SIN before the PR Card has arrived.
If you make it to the border and have identification they will let you in. It's the commercial carriers that may deny you transportation if they are not satisfied with your proof of PR.