Took Rt 2 all the way from the UP to Redmond, WA back in 2009, and it was a great trip. With the exception of a 10 mile stretch just inside the eastern Montana border, where they dug up the entire road to convert it to 4 lanes, and the track lay across soft sand where you took your life in your hands if you deviated off the twin ribbons pounded down by previous wagons, it was a great road all the way. I'm sure they've finished that stretch by now, but 15 days should do ya. We took 14, and made it with ease.
I see that Cranbrook is west of Glacier NP, and the Waterton Lakes NP in Canada, so of course you must see those. The Upper Peninsula is chock full of beautiful little rivers and waterfalls, you might want to take a peek at a few of those.