Okay, so, no kids changes a few things. I still would not write off a half day in Dubuque at the Mississippi River Museum followed up by a half day at the Field of Dreams.
I think a day I’m Des Moines is a must. The State Capitol is decent tour, and just the grounds in general. Lots of monuments and stuff to look at. In the Summer you won’t have politicians running around, and it’s a relaxing tour. The arboretum is a solid 2.5 hour tour. I also happen to love the Pappajohn outdoor art exhibit. Go in the morning or early evening after a nice dinner downtown. It’s best to tour at those times to avoid the heat, and I think several of the pieces view radically differently in waning light. The cool thing is you can walk around the pieces and see them from different angles and different lighting.
If by chance they are in western iowa they need to get off the interstate and see the Loess Hills. Those are some of the more unique Iowa things I can think of. Unless, by some stroke of luck, they are here during the State Fair. That cannot be missed.