you can say " Can I have a medium(small, large) drink" instead of " I want a coca(扣可)", because you can choose any drink you like by yourself. I've never been to the Canadian Mcdonald's before, but i think it's more similar with the America's.
My favorite is Chicken Nuggets and fries with some extra ketchup or barbecue sause.
But the kids would like Mcflurry.
Do you have language difficulties at McDonalds?