If you decide to take I-90 all the way to Seattle, there is lots to see along the way (Mount Rushmore, etc.). I would highly reccommend that you stop in Coeur d'Alene, Idaho (if you take I-90 you will pass right through it just before you reach Spokane, WA). It is about 5 to 6 hours away from Seattle, and a good safe place to stop for the night before you make the last leg of your journey. Lake Coeur d'Alene is has been called one of the five most beautiful alpine lakes in the world. There is a floating boardwalk that is almost 3 quarters of a mile long that offers a good view of the lake and surrounding mountains. There are lots of great restaurants and places to shop. CDA is a very tourist friendly community with a number of fantasitic golf courses and ski resorts.