Highway 20 across NE is a great way to get west from Iowa. Starts in the Sand Hills. Passes along the Niobrara, a National Wild & Scenic river. Outfitters in Valentine will handle your equipment and shuttle needs, makes a great day on the water. Smith Falls State Park is NE's newest, the falls are 63' high. Special place on the Niobrara with a great little campground.
Next you pass by the hometown of Mari Sandoz, she wrote the definitive biography of Crazy Horse. Don't miss the Museum of the Fur Trade in Chadron, right on the highway. This is really the story of how the west was explored. Fur traders, particularly the French, were all over the place long before Lewis & Clark. Museum has a wealth of information, lots of items on display and a fantastic bookstore. Its a steal as admission is only $5.
Close by to the north but 20 tough miles is Toadstool Geologic Park. Hoodoos like you see at Goblin Valley State Park in southern Utah.
You'll be passing by National Forests most of the way, they end near Fort Robinson State Park. You can explore the Soldier Creek Wilderness but it will take some time. Fort Rob only purpose for me was to remind me that winners write the history. The real story of this place is its ugly place in our history of dealing with the tribes for which this is home. We arranged for the murder of Crazy Horse here. But that story is completly untold.
We've crossed on US20 in both directions and always find nature and history close at hand. May not be the mountains or ocean but its serene and beautiful in its own way.
PS, shout out for Agate Fossil Beds Natl Monument. About 50 mile south of Fort Rob. Contains much of Red Cloud & his families prized possessions. This has to be the finest collection of Lakato items that the NPS owns but likely its simply the finest we have. Its also a place where paleontologists unearthed full skeletons of extinct mammals.
If you are headed to Rocky Mountain NP a great route is to continue south to Scotts Bluff, then on thru Pawnee National Grassland, yes there is such a thing as a Natl Grassland! Its an easy way to get to US 34 which becomes Trail Ridge Road but only after heading up the Big Thompson Canyon to Estes Park.