Driving is your best bet. The cost to fly into and out of Des Moines is very high.
There is a public bus system but I have never taken it. I have never had any real problem dirving around. Downtown has a very nice skywalk system.
If you live less than 8 hours away, don't fly. Airfares to and from Des Moines are high. Often better to fly into Omaha and make the 2 hour drive.
Limited public transportation outside of the downtown area, especially during weekends. Rent a car!