With almost 130 acres of land, Washington Park is by far the best city park I have yet to encounter. Highlights include The Oregon Zoo, World Forestry Center, the Portland Children's Museum, miles of hiking trails, the Hoyt Arboretum, and my favorites, the International Rose Test Garden and Japanese Garden.
The rose garden has an unbelievable view of the city of Portland and, if you visit while the flowers are in bloom, you have the opportunity to view over 500 varieties of roses (and smell as many as you can without passing out).
The Japanese garden is a required site to visit, as far as I'm concerned. The Portland Japanese Garden is known as the most authentic Japanese garden outside of Japan. With over 5 acres exhibiting 5 styles, it is easy to lose hours there. If you are an artist, I recommend bringing your sketchbook. On a side note, the Classical Chinese Garden downtown is also wonderful to visit.
Hopefully by this evening, I will be able to post photos from our trip so you can see how these gardens can really transport you.