Glacier, what we did in 2016:
Day 1: We drove the 100+ miles from Great Falls to our sweet motel in East Glacier Village, checked in, grabbed a bite of lunch, and headed for the Two Medicine region of the park. You can read about it here if you'd prefer, but we did two hikes in that area -- Running Eagle Falls (more people, better falls to see) and Apistoki Falls (less people, falls were pretty much a trickle).
Day 2: Keep reading from the link above, but basically that was the day we did Going to the Sun Road in both directions. Our original plan had been to go west on Going to the Sun, and east on US-2.
Day 3: On the way up to Waterton Lakes in Canada, we stopped at the Many Glacier region of the park. You can read about it from the link above, if you'd like.