Des Moines is a driving city, and almost every visitor ends up renting a car. The downtown skywalk system — four miles of enclosed, second-floor walkways linking most major buildings — is the obvious move in winter or during a downpour, and is the largest such system in the United States. Within walkable downtown and the East Village, you don't need a car at all.
DART runs the public bus system; it is functional rather than convenient, with a downtown shuttle (the D-Line) running free during business hours. Bcycle bike-share stations cluster around downtown, the riverfront, and Drake University, and the Principal Riverwalk and Neal Smith Trail (running 26 miles to Ankeny) are good cycling. Rideshare is reliable downtown and at the airport; less so in the outer suburbs.