By may, rt 20 should be open, it's gorgeous.

Take i5 past Mt Vernon to rt 20, go to Diablo lake,, head across to twisp, then down 97 to vantage, then across 90 to the potholes/Othello. That's maybe three days if you do lots of stuff along the way. Also, you could take a little detour to Coulee city, if you like huge dams. The it depends if you want the wide open spaces or the mountains. Personally I'd head down to Walla Walla and do some touristy stuff

Never thought about it, but there's waymore than you can do in a week, especially if you like to take breaks and do stuff.