Hi John,
When we do the north west, we do it clockwise. I'm going to leave a lot of places out but bear with me. Start in the southern part, work up to San Francisco, Mount St. Helen's, etc. Go east through Washington state to Craters of the Moon NP, north to Glacier, etc. This puts you at Glacier later in the summer. All roads and hiking trails should be open. If you are able to hike, the hike to Iceberg Lake and Grinnell Glacier are day trips (one day each) you will never forget. When crossing South Dakota consider stopping at exit 131 on I90. This is where the Minute Man Missile National Historic Site is located. You'll need reservations to take the tour. Go to
Minuteman Missile National Historic Site (U.S. National Park Service) and read all about it. There is too much to see and do on your trip. Just in South Dakota, one could spend a month along I90 (with a side trips to Mt. Rushmore, Crazy-horse Memorial, Mammoth Site in Hot Springs, etc.) The geographical center of the USA is a few miles north of Belle Fourche, SD. If you go, stop in Belle Fourche, at the tourist info center, and ask where it is. Unless you do this, it is almost impossible to find.