Mid West Trip this Spring / How early can you visit

If you want to see some notable Midwest points of interest, there are many depending on your route. For example, Lincoln's home and library (Springfield, IL), the Durham Museum (Omaha), etc. Most National Parks and Monuments tend to be historical in nature across the Plains (battle fields, pioneering, Lewis and Clark) until you hit the Black Hills or the Rockies. They are open year-round. What kinds of things are you hoping to see/do? What is your intended route?