From Des Moines to Cedar Rapids, your best route would be I-80 east to I-380 north. It is largely flat, with some rolling hills.

I-80 is a major highway and stays open except in the harshest conditions.
Unless it is snowing or has snowed in the past few hours, your drive should be trouble-free. I don't see any major systems moving into Iowa in the next few days, so you should be fine.