Welcome to the RTA Forum!

I agree that you probably will not be able to rent in the US and return a car in Canada. I'd also expect that you would end up paying significantly more in airfare trying to do a one way flight back from Canada. You could still visit Vancouver, but I think you'll find its much easier and cheaper to fly home from SeaTac.

As far as what to do, there's no shortage of things, and narrowing it down will likely be the toughest part. Deciding how much of the trip you'll spend on the coast vs. in the mountains or in cities will be the tough part. To help get you started I'd look around at other threads, like this one, this one, and these ones too.