Seattle is a wonderful location from which to visit four different national parks (North Cascades, Rainier, Olympic, and Mount St. Helens), and countless state parks all with an endless supply of hiking trails. Within an hour or two, you can be hiking until your hearts content. Do a portion of the Pacific Crest Trail, or one of the many other mountains, valleys, rivers and volcanoes. The Pacific Northwest is an outdoors man's paradise. My favorite area to hike was the North Cascades, either in the National Park or one of the many state. Before you go, check out one of the local bookstores for your type of hiking trails.
There are a variety of books highlighting suitability to your needs and desires; kids, pets, elevation gain, and length. If you want to hike in the National Park, check with the visitor’s center, otherwise go with information ahead of time. If you’re not in a park, many of the trail heads require you to pay a fee for parking. You can buy this at one of the local grocery stores near your trail, Northwest Trail Pass, its $5 for a day and well worth avoiding the ticket. A few of my favorite hikes include: Little Giant’s Pass and many along Route 2. Keep in mind the setting sun, start early, and bring water, food, and emergency supplies, hike with a friend. Remember you’re going into the wilderness, beware of plant and animal danger and direction.