I84 is actually one of the scenic routes across eastern Oregon. Between Pendelton and Ontario it crosses the Blues.
The area around the I84/82 junction is flat and barren, though Hermiston is a major agricultural center. It also gets flat and straight close to Ontario, with increasing traffic as it passes Boise.
Twin Falls is a nice stop off of I84. Idaho into Utah is a broad open upland, windy, without any settlements.
US97 climbs out of the Columbia valley in dry open country. It becomes more settled around Madras. South of Bend it crosses open Ponderosa Pine country.
US20 east of Bend crosses an open dry country (north side of Christmas Valley). At Burns you could drive south past birding lakes (and the recent wildlife refuge occupation) to Steens Mtn and beyond into Nevada. This one of the more scenic areas in SE Oregon, especially if you are a birder.
US26 stays in the valleys of John Day country, best known for is fossil bed monument.
OR31 takes a diagonal to US385 past volcanic features, bounded on one side by an escarptment, and salt lake on the other. This is called the Outback scenic byway. This part of Oregon is an extension of the basin-and-range region of Nevada.
OR140 is one of the more remote roads in the country. You might have problems finding gas between US395 (Lake City) and I80 in NV.
In September the higher roads in Utah should have fall colors, especially aspens.