Nice! My in-laws live in Nova Scotia, about an hour from Halifax, and I have been there many times. It reminds me a lot of England. My sister-in-law is in Montreal, Quebec, which is also beautiful.
Having never stayed at a motel in Canada (whenever we go, we always stay with my wife's family), I can't say from experience if you can just turn up but I'd be 95% sure of it. Just remember that, like the U.S., there can be very long stretches of highway in Canada without a hotel in sight for miles and miles, so I would at least have a rough plan in place as to the town you will stay in, even if you don't know the motel yet. Also bear in mind that August will be a peak month tourism-wise so it's more likely that some places will have no vacancies.