If you have three weeks or a month, that is plenty of time to access ANY of the 49 mainland United States (yes, even Alaska), and most of Canada. Don't necessarily limit yourself to 1500 miles. You have the opportunity of a lifetime here; now is not the time to set limits on it.
What kinds of things do you like to do? This would narrow it down a bit; this country is HUGE.
One thing I'd recommend is a AAA membership ((http://www.aaa.com
), for a few reasons. One, their roadside assistance services have saved us a few times. Two, their maps and TourBooks are great sources of info, and have great lists of attractions and lodging for just about everywhere. Get TourBooks for all states/provinces you might visit, and research them. Start to plan things out.
There are MANY incredible scenic wonders in the west if you like nature, the outdoors, and National Parks. There's Yellowstone, the Grand Canyon, the list goes on. There are things to see all over this country. If you want to try driving to Alaska, it's actually a very nice ride, very comfortable through the summer; it's like nothing else you'll ever see.
Really, I have no idea where to start, there's just so much out there. Once again, what kinds of things would you like to see and do? If you start to narrow it down a bit, I can start to suggest more specific things.
Exciting stuff! Enjoy it!!