In central Illinois the banks of the river are flat for miles around. But around the Palisades State Park they rise to a rolling plateau. The park is one of the best ones in Illinois. From there north to Minneapolis the river is in a valley a couple of hundred feet deep.
SW Wisconsin is called the Driftless area, an area of hills that were not scoured flat by the big glaciers. There are some towns in the area (and in NE Illinois) with a mining history (especially lead), and now are tourist attractions. On the other hand central Wisconsin around Wisconsin Dells is very flat, a former glacial lake bed. Except the Dells themselves which are scenic, and very touristy.
The main highway runs on the Illinois/WI side as far as I90. From there the main highway (with 4 lanes) runs on the Minnesota side. There is also a good state highway on the Wisconsin side, though I think it strays from the river more.
The state boundary follows the St Croix north, while the Mississippi turns west to the Twin Cities. I don't think there's a distinct 'river road' along the St Croix, though certainly there are access points and crossings.
Check the respective state park sites for scenery. Generally those preserve the most scenic spots. There are wildlife preserves along the Mississippi, and state (and national?) forests in northern Wisconsin and the Michigan UP.