Omaha is a beautiful city with an excellent zoo. The Henry Doorly zoo should be a hit with the kids.
The best activity in Des Moines is the Iowa State Fair. It runs from Aug 10-20. You must stop to experience it if you are passing through during those dates.
The Devonian Fossil Gorge in Iowa City is a fun, short stop if it is not too hot.
There are short paddlewheel boat trips on the Mississippi out of the quaint town of Davenport Iowa.
One of our favorite hikes in Yellowstone was alongside a river to some hidden hot pools alongside a cold river. We saw herds of antelope along the way. The kids will never forget it, but make sure they don't put their heads under the water when in a hot spring.