My family is from Portland and I visit every few months, I agree with all the above suggestions. Love Multnomah Falls, love Saturday Market, the rose gardens and I'll add in the Zoo.
A few of the bridges are stunning, my favorite is the St. Johns Bridge in N. Portland. Its beautiful, the architecture just amazing. There is a park that is under the bridge called Cathedral Park where you will see for yourself why it is called Cathedral Park.
Cannon Beach, home to Haystack Rock is a beautiful little coastal town full of shops, galleries etc. You can continue south to Tillamook and visit the cheese factory.
A day trip to the Mt. St. Helens observation points has been on my to-do list for a long time. It always seems to have uncooperative weather when I am there and I have yet to make it out to see the volcano.
One of my favorite off the beaten paths places to eat is Pine State Biscuits, sooooo good!
http://www.pinestatebiscuits.com/
We also love OMSI
http://www.omsi.edu/
It's not just a kids museum. I noticed they are hosting the "Mummies of the World" exhibit beginning this week. My daughter and I saw this exhibit in San Antonio, TX in January and really enjoyed it. Fascinating
I am very fond of McMennamins Pub's
http://www.mcmenamins.com/
The food is just okay, but the beer is delicious and the atmosphere is like none other I have seen. Each location is different and fun! I love the Ruby Ale
Another fun food adventure is the food carts located downtown in the Pearl District. Lots of yummy stuff to sample.
If you have children in you life, the best toy store on the planet is also downtown near Pioneer Sq.
http://www.finneganstoys.com/ If you can find it at Toys R Us chances are you won't find it at Finnegans. They have wonderful toys, dolls, science stuff etc. My kids have outgrown it but my Dad and I spent an hour in there last year just browsing!