How to show proof of vaccination for attractions/restaurants as a Canadian in New York City?

A lot can change between now and next April, and you should make sure to re-check all the requirements closer to the time of your travel, but currently, among the acceptable proofs of vaccination is:

NYC Vaccination Record or other official immunization record, including from your health care provider. A digital photo or photocopy of this card is also acceptable.

Your Canadian vaccination card is an official immunization record and should be accepted by venues. You can also consider the official "NYC COVID Safe App", but it's just a specialized photo album and doesn't seem to provide much benefit.

As a practical matter, if you are polite and show a paper (or digital photograph of a paper) that looks like it is probably a proof of vaccination, possibly explaining "I'm Canadian" to anybody who looks momentarily puzzled, you are almost certain to be allowed access to the venues you want to go to. As you know, New York City sees a lot of domestic and international tourism, and business generally want you to be their customer if they can do so while following the law and protecting their employees' and other customers' health. The law requires that you also show an ID that matches the name and date of birth on your vaccination proof, though in practice not all venues seem to ask for this.

I do have a New York Excelsior Pass, and regularly show it as my proof of vaccination, and nobody has ever scanned its QR code (it is my understanding that some very large venues may do so, but I have not experienced it personally) so I would not worry too much about Canadian QR codes using an incompatible system. In any event many visitors, even day-trippers or daily commuters from neighboring states, are not eligible for the Excelsior Pass and still need to be allowed in to places.