Much will depend on what time you get on the road as you won't have a whole lot of free time, but you will have two relaxing days of travel on the way. I would probably look at taking US2 through Okanogan - Wenatchee National Forest and stop over in Coeur d'Alene for the night which would give you a comfortable run to West Yellowstone. On the way home, if you decided to go through the Tetons, [recommended] you could stop in Jackson WY and then Boise with time to visit Craters of the Moon or Shoshone Falls and Snake River canyon on route.