If you have time to visit only one or two places during your Northwest stay, these should be among your top choices. In addition, there are many less famous places that are well worth seeking out. Many are best done as a multi-day trip. For example, a good 2-day trip could combine visits to Mt. St. Helens and Mt. Rainier, with lodging overnight somewhere in between the two parks. Likewise, North Cascades National Park can be best explored as a 2-day loop, going over Hwy 20 and returning via Hwy 2, or vice versa.
Most campgrounds also charge a fee.
Visit this website for more information. Washington is one of the premier hiking states in the country. Although you can get a good taste of Washington from the road, the only way to experience the wild areas meaningfully is to get out of your car onto a trail. Most major attractions have wheelchair accessible trails. Trail notations below include round-trip distances and elevation gain, where applicable.