Rawlins to Boise

CloudDriver's route is the most direct without Interstates - have driven all of those roads with no issue. US20 drops in altitude from Fairfield to Mountain Home and winds through some canyons, but no problem in the summer with a trailer/mh.

For the scenery, I might go north on US 287/26 from Lander to Moran in Grand Teton National Park, and then south to Hoback, but there is definitely more grade involved.

The following avoids interstates as much as possible:

Route 287 north out of Rawlins to Muddy Gap and on to join with route 28 a bit south of Lander. At Farson head north on route 191 to route 26 at Hobart Junction. Go south on 26 to Alpine Junction near the Palisades reservoir and continue on 26 to Idaho Falls. Take route 20 west to Arco then route 26/20 to Carey. At Carey route 20 splits off to take you to Mountain Home. From there a section of I84 looks the best to get to Boise.

I have driven all of these roads except the section of 287 between Rawlins and Muddy Gap and the roads beyond Craters of the Moon west of Arco.

Don't recall any excessively steep or long grades.

Craters of the Moon is worth a stop, as is the old mining town of South Pass City a couple of miles of good gravel road off route 28.