US 6/191 from Provo is the most logical deviation from a strict freeway route.
One time I left I15 at Ogden, took I84/80 into the mountains, and then US32/35 across the south west side of them to Duchesene, and US191 south. This had more passes, and some nice fall Aspen colors.
I've driven the US12/ US95 route through Lewiston to Boise, as part of an Idaho trip, It's nice, but I'm not sure I'd use it as an alternative to I84 across NE Oregon. That part of 84 is somewhat mountainous and scenic. It's the ID part of 84 that tends to be boring. Boise to Twin Falls crosses a rather flat and dry Snake River plain. 84 crossing into Utah is a higher broad crossing, potentially windy.
You could also bypass Salt Lake City by taking US93 south at Twin Falls, as far as US50.
Look at map with good terrain information. That shows you how the freeways tend to stay in the valleys and plains between the mountains. With the exception of major passes across the Cascades and Rockies (I90), and some what lower Blues in NE Oregon. NE Utah is also high, which is great if you are skiing, less so if you are driving a big RV.