What would constitute an unpleasant surprise? You will encounter some grades in the Idaho mountains, but nothing difficult.
ID17 connects ID21 to 55. That will take you to the McCall area, bypassing Boise. I've only seen Boise from I84, so can't say much about it - except that the surrounding area is flat irrigate farmland.
75,21,17 wind through the core of the south Idaho mountains, mostly following river valleys, with numerous hot springs and small campgrounds.
McCall is a lake vacation land for Boise residents. So the drive between the 2 can be busy on summer weekends.
North of New Meadows, the landscape is dominated by the Salmon, Snake and Clearwater river valleys, with uplands between. Most of the grades climb in and out of those valleys.
Another drive in the Grangeville area is ID14 up the South Fork Clearwater to Elk City. In a suitable vehicle (not an rv), this continues east as the Macgruder Corridor between two major wilderness areas all the way to US93 in Montana.
https://picasaweb.google.com/105634639868707713567/IdahoThis is an album of photos from a May loop around Idaho several years ago. It was a cool wet spring, so we encountered some snow (north of McCall, Lost Trail Pass, and back Washington). We entered at Lewiston and looped around to Missoula and back through northern Idaho.