2 from alliance ne to i think it's either 97 or 83 up to valentine ne is nice. that stretch of 2 has a lot of sweepers and plenty of trains to look at. 20 from there through iowa is okay, definitely better than I80 or I90. mid july 18 through western iowa was a construction nightmare, not sure of condition now ( though i would imagine it's finished up).
44 across SD was beautiful, particularly at the missouri river, there's a nice little pull off up on a hill there.
i do St. Louis to Cedar Rapids often and really enjoy 79 up to hannibal, hit some decent alphabet roads off that route one year when flooding detoured me, don't recall the letters though. past hannibal there isn't much to see through iowa until maybe burlington, which has some decent river views and snake alley ( silly little curvy short brick downhill road).
the river road through the quad cities is a pain, i'd avoid it if for some reason you come up through iowa.
poopy's in savanah illinois is a bit of a giggle for a biker bar, worth a stop once anyway, particularly on a weekend. Bellevue on the Iowa side, just up the river a bit has some nice riverviews and a nifty overlook of the town from the state park just to the south.
dubuque then north is probably the most notable portion of the iowa great river road, good stuff with nice overlooks, nothing tricky but decent sweepers and plenty of bluffs. C9Y is about the only somewhat twisty stretch, but it's fun.
overlooks from that stretch:
Balltown
Pikes Peak Park (Mcgreggor/marquette)
Mt Hosmer (Lansing, IA)
there is also a great little meat market in new albin that has fantastic beef jerky and a sweet pork jerky that is amazing.
halfway up c9y is a ferry across the mighty miss to cassville, wi, but i think it is only open on weekends this time of year and is 8 bucks per bike. i usually just stick to the iowa side.
as far north of Prairie Du Chein i think the wisconsin side is more scenic, but the iowa side has far less traffic ( plus mt hosmer and the meat market) there are bridges in prairie, lansing and la crescent. there are some decent roads in that part of MN too, but i don't remember the numbers i took.
as mentioned, all of sw wisconsin is fun once you hit the alphabet roads, but if your goal is to have decent scenery and roads while making time, these bits are pretty good.
sorry for the rambling