The route suggested by Google Maps is about the best way without going way out of your way. Very pretty too! Interstate 90 to 395 South West on 30/84 over to interstate 5 South to Coburn
I would highly suggest bypassing Portland by using South 205 around the city. Interstate 5 through Portland can be a nailbiter!
Easy Sailing as it is all good roads and light traffic excluding rush hours around Spokane & Portland.
I90 to Ritzville, 395 south to Tri Cities, I82 south to I84 to Portland area, I 205 south to I5 south. Not really much to look at except miles and miles of freeways until you get into the Columbia River gorge where you can see a lot of nice waterfalls.