Everyone says we should see Glacier Natl Park, but how many mountains do we need to see? If you have traveled this route, or parts of it, all advice is welcome.
I'm firmly in the "how many mountains do we need to see" camp. And one thing I learned about Glacier is that it was formed by glaciers and is not made up of glaciers. So it's really just mountains.
However, if you're on I-90 anyway, you'll be pretty near Glacier, and could do a detour that isn't an out-and-back. And it's very likely a place you're not going to be near again. So I'd say do it. Since you're in a car, you can get to the park in the afternoon and poke around a little, and the next day you could drive the Going to the Sun Road and out the other side of the park and continue to Yellowstone if you want just a taste.
I'd recommend also going to the Many Glacier area if you have time because the setting, with the lake, is so pretty. In fact, you could stay in the hotel there after driving the Going to the Sun Road, and then leave the park the next day.
When we were there, we were in an RV and had to leave it in an RV park in Columbia Falls while touring Glacier. Driving a car and staying in hotels along the way would have been much more convenient. But we still managed to do a hike from the Many Glacier area up to an actual glacier--about 10 miles round trip. But you'll see lots of glaciers on your cruise; I've done a glacier cruise to Alaska and I've done a hike to touch a glacier and I can't say my life is greatly enhanced by actually touching one. However, our walk took us through a big herd of sheep and they were fighting right near us, and the clacking of their horns was LOUD. When we came down there were all these people looking at the sheep with binoculars and I was thinking, "Heh. We were just up there."
Also, on the Going to the Sun Road there was a fog bank in the valley that we drove above that was super cool. But of course you never know if you'll get a cool fog bank or encounter a herd of sheep. So maybe a one-day visit to some place you're pretty sure you'll never get to again is sufficient.