Portland is blessed with one of the largest park systems in the U.S. Two of the best to check out are Washington Park and Forest Park .
In Washington Park - a well developed, planned place - you can stroll through the famous Portland Rose Garden (free), find inner bliss at the Japanese Gardens, be a monkey at the zoo, learn about the trees at Hoyt Arboretum, visit a mansion, and bask near the many fountains, statues, and playgrounds. The park also includes a Childrens Museum, a Lewis and Clark memorial (one of my favorite places), a great Vietnam memorial, and a Holocaust memorial. Throughout the park are amazing vistas of downtown Portland and Mt. Hood and Mt. St. Helens.
Forest Park is an undeveloped natural park that has over 74 miles of trails for bikers, hikers and horseback riders. It has the distinct honor of being the largest forested park within city limits in the U.S. Explore the Wildwood Trail for a long, enjoyable hike. Trail maps are available here. Check out the Friends of Forest Park for fun activities and volunteer oportunities.