Marshalltown,IA. Swift smells real good this time of year
Stayed down by Michigan Ave (shopping for the wife)
Spent plenty of time at Navy Pier for the kids, along with visits to the Field Museum and Shedd Aquarium
Cubs game at Wrigley
Unique restaurants, great food
We go to the Lake of the Ozarks for at least a week every year. Obviously love the place. If travel time is part of the three days, that's not enough time to enjoy the Lake if driving from Iowa. It's a 6 hour drive from where we live right on the Iowa side of the Mo. border. If you like to boat, jet ski, ski, bar hop on the water and all things associated with water activities Lake of the Ozarks is the best. 1500 mile shore line with private development allowed. Nothing against Okoboji, but its a puddle in comparison.
If you like camping and canoeing, try the Yellow River Forest in NE Iowa (Allamakee County, I think). It is some of the prettiest scenery in Iowa.
Okoboji is cool, but the Ozarks is on a whole different level.
Chicago. St. Louis.
Black Hills if you have 5 days.
Apple River if you like naked, tatooed, and flashing ladies.
HoneyCreek Resort near Ottumwa! Run by the State! (that was a joke).
You could take 100 3 day weekend trips to Chicagoland and do something different every time. I-88 is as smooth as Lindsay Vonn's belly.
Fort Lauderdodge if you like naked, tatooed, and flashing homeless dudes.
Any creek in Iowa with arrowhead or geodes....and a farmer kind enough to give you permission to look for either.
Good fun all around - get to learn about nature, bugs (including mosquitoes and ticks), geology, play on mud slides, etc. Throw in a picnick with a campfire, makes for a great day.