The State Historical Museum of Iowa is so much fun! It might sound crazy to take a 1.5 year old to a museum, but I loved taking my son here when he was that age and my daughter had just as much fun. This is a great spot to enjoy and learn about nature/outdoors-y things without actually being outdoors. Admission and parking are free and there is a ton to look at; it is stroller friendly with lots of open space and stairs for toddlers to climb.
Editing to update – they have a new exhibit just for little Iowans, see our visit there!