I am planning a car trip up to Seattle to see friends and am writing to ask for advice on good scenic alternatives to the all-Interstate route (I-135, I-70, I-25, I-80, I-84, I-82, I-90). Casual inspection of possible alternate routes in Google Maps tells me that it is very hard to beat in terms of distance travelled, but I don't want to drive it the whole way. The speed limits are too high: my little car has only four gears and is ferociously thirsty at 75 MPH. Moreover, about half of the route, including Wichita to Denver (I-135, I-70) and Pendleton to Tremonton (I-84), is already familiar from previous trips.
Right now I am considering alternate two-lane routes between Denver and Rawlins (US 40 and SR 125 in Colorado, SR 230 and SR 130 in Wyoming), and between Little America and Tremonton (US 30/SR 30 in Wyoming and Utah), but I am stumped for good alternatives to I-84 in Idaho.
(See map.) Is it workable to detour as far south as the Owyhee country (which I haven't visited), or as far north as Hells Canyon (ditto)?
In Washington state I don't have a pressing need for advice on how to avoid I-82 since I haven't driven it and WSDOT SRView suggests it is actually quite scenic climbing out of the Columbia River valley. However, I am open to recommendations as to Cascade passes. I have only ever done SR 20, parts of US 12 and SR 410 (I visited Mount Rainier), and never I-90 (Snoqualmie) or US 2 (Stevens).
In terms of added mileage, I think I would be prepared to accept about 10% over the all-Interstate route, and possibly more if the detours had good scenic value. Google Maps quotes 1823 miles for the direct all-Interstate route, 1879 miles for a calculated route that passes through I-25's north end in Wyoming, and 2049 miles for another calculated route that passes through I-70's west end in Utah.
I'd be much obliged for any advice.