Lime Creek Nature Center in Mason City has hiking trails, a natural playscape and butterfly garden. The nature center is open in the afternoon, and unfortunately we visited in the morning, so we didn't get a chance to check it out.
My kids enjoyed the Smart Trail informational videos as we hiked. They were short and informative. Definitely worth taking the time to watch as you hike. Here's a Smart Trail map.
My kids were not impressed with the natural playscape, but that's only because they've been to so many bigger ones in Iowa. They still had fun playing. The playscape is next to a beautiful butterfly garden, and there's also a shelter with picnic tables.