I love the Oregon Zoo. It's set in a beautiful area of Washington Park in Portland Oregon. There are lots of trees and it's a great setting.
We have a yearly pass, which really pays for itself in 2 or 3 visits. We try and get there just a few minutes before it opens and have often had the zoo virtually to ourselves for 30 minutes or so until attendance picks up. It's wonderful.
There are plenty of animals and they are expanding and adding exhibits all the time. Recently, we watched the bird show and it was fun, informative and impressive. I was amazed at the majestic and varied birds they had, many of whom were rescued or could not survive in the wild.
Food at the zoo is a little pricey, but they have fun kids meals with an animal toy, if you are in a generous mood. Otherwise, pack a lunch and eat out on the big lawn. It makes for a really fun day.