I live in Indianola. I went to Iowa State in Ames. When I was in school, we used to go terrace jumping... probably wasn't the most legal thing to do. You know what they say, young dumb and full...
I don't really know of any place around there.
I live in Iowa City after college and there is a great place in Solon, IA called "80 acres." Lots of mud trails and hills. Weekends are crazy when it gets nice out. That is only an 1-1/2 away.
Good luck