1) National WWI Museum. Everything else is tied for #2 or lower.
Steamship Arabia museum
Truman Library
Truman Home
BBQ
Train Station
Farm Museum west of town (name escapes me, near Bonner Springs)
Never been to Ft. Levenworth, but it's suppsed to have some cool stuff
Battle of Westport sites scattered throughout town (Civil War)
Beer Kitchen
Atchison, Kansas, is a pretty town, Amelia Earhart childhood home, neat old houses, museums