My family loves Arnold's Park. We get up to Okoboji at least once a month. If you know you are going to be there often, or more than 2 days, I suggest buying a summer pass. It will run you $65 for the entire summer. (single day pass is $25ish/ 2 day pass is $39ish- depending on the height of your children.) The go-carts and putt-putt are an additional cost.
There are rides for all ages and has the only wooden roller coaster left in the state of Iowa. I highly suggest the younger the children visit early in the day/evening you take them. The night time crowd is more 12-16 year olds. My kids have a BLAST every time they go, even if we can only make it down there for an hour or so.
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