The Des Moines Art Center is one of the best small museums in the Midwest, both for its collection and for the building it lives in. The permanent collection runs from Edward Hopper and Georgia O'Keeffe through Francis Bacon, Mary Cassatt, and Jasper Johns; contemporary holdings include strong pieces by Andy Goldsworthy, Olafur Eliasson, and Ai Weiwei. The galleries are intimate, so you can spend a focused two hours and feel like you've actually seen everything.
Admission is free. The museum sits on the edge of Greenwood Park, where the Pappajohn name shows up again on the Rose Garden, and the cafe inside is a genuine lunch spot rather than a desultory afterthought. Allow time to wander the sculpture park out front and the Pei-designed central court, which doubles as an event space and is worth ten minutes on its own.