Immerse yourself in the wonders of the prehistoric world! You might think today's predators are pretty fierce but just wait until you see those from the past. From the massive giganotosaurus to the cunning troodon, dinosaur predators were skilled at finding their prey and surviving in a hostile world.

If you have young children and live in Portland, you should definitely belong to the zoo.  I go back and forth about my feelings for animals being caged up for our enjoyment, but as far as zoos go, we have a really nice one with grounds that attempt to mimic the animals' natural habitat.  I just took my boys there other day and barely looked at any animal displays - we came to play in the sandbox and ride the train.  There aren't many sandboxes at Portland Parks, but the zoo has a great one.  It is covered so you can go there on a misty day and stay somewhat dry or it's a nice place to stay out of the sun on a hot summer day.  There are lots of digging toys there - buckets, shovels, dumptrucks, etc.  There area couple tables too so you can stop and take a snack break here while at the zoo.  My son also loves watching the tidepool across the way that shoots water high in the air every few minutes.
The train has a short zoo loop and a longer loop through Washington Park.  You are able to see some animals (elephants, wolves, birds) from the train.  Check the website to see when it is running because right now it only runs on the weekends.  After Memorial Day it changes to daily runs.  
It's worth having a membership so that you can come for a short time and see/do a few things and then go.  You also get a discount on train rides!  We often get to a few of the concerts they have during the summer. 
We also often take MAX which has a stop that drops you off right at the zoo.  Very convenient!