Bring plenty of clothes and good raingear! Plan to dress in layers too, because it can change quickly. Last year fourth season, I had basically everything on I had with me in the morning and was still cold. But by the time I was dressing out the gobbler mid-morning, I was in shirt sleeves. Spring in Iowa (like the other three seasons) is unpredictable, but well worth the trip, especially if you can stay warm and dry.
Good luck -- let us know how it goes!
Threebeards