Depends on what kind of terrain you want and how far you're willing to drive.
There's a small trail/park south of Council Bluffs. I think it's called Riverside Park. It's just across the Veteran's Memorial Bridge. No side-by-sides allowed the last time I checked and Iowa requires a license/sticker for your ATV.
About three hours away is Tuttle Creek ORV Park just north of Manhattan, KS about 20 minutes. Lots of varied terrain.