Up Montana way, and depending on where you exit Utah or Wyoming, you can consider a few short side trips/stopovers:
If exiting Wyoming on the east of the Rockies, you can look at some remote campground camping in the Custer NF west of Broadus. That could follow a visit to Devil's Tower NM just on the Wyoming side of the border. North of there is a whole lot of high plains with some badlands mixed in, making it possible to consider the National Grasslands tracts and Theodore Roosevelt NP in western North Dakota. More due north from where I-90 crosses from Wyoming into Montana is the Missouri Breaks NM (NP?) and some patches of low mountains in the Lewiston and Rocky Boy areas. You can even cross the Missouri River on a cable ferry at McClelland or Virgelle.
If along the I-15 corridor in eastern Idaho, Several "jug handle" or out-and-back side trips are worth consideration. Craters of the Moon NM is a great place to take kids to (though hotter than the hinges of Heck in early July). You can also loop to the west up ID 28 to Tendoy, turn up a graded gravel road, pass by Sharkey's Hot Spring for a soak, and cross into Montana at Lemhi Pass, where Lewis and Clark crossed in 1805. Descending the MT side takes you right past Bannack, MT enroute back to I-15 at Dillon. An eastward loop off of I-15 starting at the Idaho-Montana state line (Monida, MT) can take you into the vast Centennial Valley for 25-30 miles, thence north into either the Blacktail Creek valley to loop back to Dillon, the Ruby River valley to emerge at Virginia City, MT, or up and along the breathtaking Gravelly Range Road for a 35 mile cruise mostly above timberline to emerge at Ennis, MT on US 287 not too far south of I-90.
Foy