It's fine 99.99% of the time no US citizen will stay in Canada illegally and they knw that.
You can stay 6 months in any 1 year period.
You can buy if you want to, just carry these 3 things
1)A rental agreement or properly ownership title from US
2)A letter from your employer stating you work in the US, or if self employed carry copies of IRS tax transcripts
3)Proof of funds, cash, credit cards , bank statements
Just keep these things in a file with you as you cross border. Most of
the time no one will ever ask to see them, if
they do you are well prepared.
Really it's not a big deal for Americans crossing into Canada as long as they have a job and a place to live back home plus no criminal record.
In terms of CRA you don't have to file anything just ignore that because your not working in Canada nor do you have any intention to, they won't ever ask. If you start renting out your propery than that's a diff story.