You put in right in town in gardiner for a scenic Class 3 run. The put in is just upstream of the bridge over the Yellowstone, there is a trail down to the river. It's a great stretch to get wet. Take out is a fishing access downstream. Then there is Yankee Jim canyon further downstream, 3 as well now, that is fun to mix it up.
If you want class 4 in the area, Beartrap canyon on the Madison is a must do.