Going to the Sun Road
The main highway going east to west (or west to east) is the 50 mile (approximate) Going to the Sun Road (Highway). If you are sightseeing, take some time to hike some of the Glacier National Park short hikes such as Trail of the Cedars, Hidden Lake Trail, or Virginia Falls or if you are fairly fit, take a day hike like the Highline Trail from the Continent Divide at Logan Pass to the Loop or any the other day hikes.
Many times you can see Mountain Goats, Bighorn Sheep, Marmots or other wildlife (once in a while a Grizzly Bear).
I love the wildflowers during certain parts of the summer especially along the Hidden Lake Trail.
Be sure to read a book or two about Hiking, Backpacking or sightseeing in Glacier National Park.