There are lots of choices - almost too many. Roughly you could choose anything within a box bounded by:
I80 and I15 on the south and east
I5 (or US101) on the west, and WA20/US2 on the north (or even Hwy3 in Canada).
In between you could take I90 to Montana, or I84/US395 to Spokane. Or US12 across Idaho to US93, and then north.
You can pickup US93 in Nevada, or take any of US97, US395, US95 north.
Instead of routes, you might think about significant geographical features (and towns), and whether you want to cross them or skip them.
the Sierra Nevadas, Nevada deserts, Oregon Cascades, Oregon Outback, Washington Cascades, Columbia Gorge, Snake River, Idaho mountains, northern Rockies are major geographic features.
Towns include Portland, Seattle/Tacoma, Spokane, Missoula.
