You can just walk up, but your car choice might not be available, and the price you get could be a lot higher than reserving with discount codes.
In the US, it's possible to make a reservation without a deposit or payment, so you could reserve one of each and decide when you get there, assuming Canada has the same setup. I suppose it's a minor pain for the company, but that's the way they allow things to be done, so it's within the rules. You might make the reservations for slightly different times, say half an hour apart, so there's less chance the company thinks one Res is a mistake.
It's a pretty place to drive around. Enjoy yourself.