If you are traveling in April then you should consider stopping in Mt Vernon for the Tulip Festival. There’s a petrified forest state park just before you cross the Columbia on I90 that is an interesting detour. On the other side of the Columbia River there’s a lookout point, Wild Horse Monument I believe, that gives you a good view of the river and is a nice place to stretch your legs. If you have the time you can detour in Montana up to Glacier National Park (I regret not being able to do this on my Yellowstone trip). Since you’ll be in the area I also suggest driving down to the Grand Tetons through the John D Rockefeller Jr Memorial Parkway. If you want to camp in the park make your reservations early, but if you don’t there are campgrounds within reasonable driving distance outside the park, there’s also a pretty nice KOA in Cody, WY (about an hour from the East Entrance). West Yellowstone is right outside the west entrance, so if you want to stay in a hotel outside the park that’s probably the best option. Also, look for the huckleberry vodka in the parks giftshop, it’s delicious! You can also get huckleberry milkshakes in MT and they are delicious!