I would spend a day in Seattle and then take a little road trip to the North Cascades National Park, Mount Rainier, Olympic Peninsula, or the San Juan Islands depending on what you're looking for. Mount Rainier will be the best for mountain views and hiking, North Cascades has beautiful scenery with mountains, cute small towns, and lakes, the Olympic Peninsula is very green and lush with some cool lakes, and the San Juans are great for water views. I don't think you can go wrong with any of them.