I'm in Calgary, have ridden to Michigan and ONtario a few times. SOme thoughts:
1) I really like the route around the North part of Lake Superior if you haven't done it. But, it is longer.
2) I prefer to take route 20 across the NOrth part of the U.S. Nice small road, not lots of traffic, small towns every once in awhile, easy riding.
3) If you do the U.S. route, you can either head across the U.P. of Michigan or down south of Lake Michigan.
4) If you can, go through Sturgess, the BLack HIlls, Devils mountain, Mt. Rushmore. AS LONG AS it isn't Sturgess week.
5) Head up through Billings Montana BUT, make sure you stop by the Custer Memorial enroute if you are ANY kind of history buff.
6) As someone else mentioned "go to the sun". (The "going to the sun road" in Glacier National Park).
7) The Salmon vally in idaho is gorgeous, get as much travel along this road as you can. I've been on it 2 or 3 times and I'd recommend it to anyone. Nice pavement, curves (not a ton but some nice ones), all along the Salmon river in a nice vally. Watch out for cows and wildlife on the road (especially the black cows at night).
8) Gas and rooms are less expensive in the U.S.
9) If you get caught speeding in the U.S. it won't hit your license for points (of course that's generally true of getting caught in any province other than your own). I also fine the U.S. police easier on me if I am going a tad fast than the Canadian ones. I always use a radar detector.