I'd also (in my perfect world) ride what remains of the Lincoln Highway and Highway 50, both coast to coast.
I always liked the idea of saying, "I'm gonna get in my car and drive to ______ today." The blank being "the ocean", "Canada", "the Mountains" or "the state line".
Same with riding, I wish I had the freedom to say: "I'm going to ride my bike to Kentucky and back this summer." Of course Mandy had actually mentioned doing just that, so maybe when the kids are a little older that will be a summer or fall adventure for our whole family.
I read about a family that rode cross country in Canada. It was a cool story and it was something that theoretically we should be able to do.