Outside of having her up to date shots, health certificate, copy of US and Canadian registration papers, a letter from the owner that the dog will be staying with me for a while - is there anything else that I am missing for a dog to get across the border to show in Canada?

TIA

You really only need a current rabies certificate. I take my show entries along as well, but I always carry those to shows regardless of which country I am in.

what "Gone Anon" said

The letter is important as the border may try to charge you tax on the dog if they think you bought it.