Powell's Books at the corner of Burnside and 10th. DS actually sets aside an entire day for this each time he goes to Portland.

The local waterfalls.
A drive to the coast to tour the Tillemook cheese factory.

The Rose Garden. And dinner at the Portland Grill. Yum!

The Cascade range offer many views. A visit to Crater Lake is wortth the side trip. Mt. Hood is attractive.
Highway 101 along the coast of Oregon from Brookings to Pacific City is beautiful

If you like gardens ....

Portland's Classical Chinese Garden in the midst of the city, takes up only about one square block but is packed full of beauty. There is a tea house if you want to stop for a rest and have a snack. It is a bit pricy at $8.50 (50 cents off with a AAA membership card) but most visitors feel it is very a worthwhile visit. It is not funded by tax dollars so an entrance fee is necessary.
However, there are several parks which are free.

Here is a link that might be useful: Classical Chinese Garden

In addition to the suggestions above (all good ideas!), check out:

Portland Japanese Garden
Oregon Zoo
Farmers Market
Saturday Market
many wineries and brewpubs

Here is a link that might be useful: Portland attractions

I will check all of these out and do map quest to them~~if you have anymore please post.

Jemdandy we went to Crater Lake 2 years ago you are right it is worth the trip, it takes your breath away!!

lunchlady, if you have the time, i'd do I5 north to san luis obispo, and cut over to the ocean and hiway 1...you just can't go north on vacation without the big sur...i'd go back over to 101 and continue into oregon. we, too, were fans of the rose gardens...you might want to stop in ashland if you're into shakespeare theater...they have great summer performances...are you into cheese? tillamook would be fun...

wish i could remember where on I5 we visited the covered bridge...

Someplace we always say we want to go back to is Hurst Castle. It is beautiful and there are several different tours.

I lived in Portland for 21 years. A wonderful place, very laid back. I would surely drive up the coast as the others mentioned. It is a very calm and fun drive.
I always enjoyed the Pittock Mansion in Portland. Mostly because, if you put your car in neutral in front of the mansion, your car will mysteriously go uphill backwards by itself. It is one of those weird things. Never was sure why that happens. A bit creepy, but really fun! A must do in Portland. Also the Saturday market is wonderful, I think it is still held under the Burnside Bridge. Lots and lots of fun to hang out in downtown.
Mt. Hood is beautiful and only 1 hours drive from Portland. Tons of little towns and pubs along the way too.
Timberline lodge on the Hood is a nice place to have lunch.
Multnomah Falls is also really beautiful, another close drive. A must see.
Tillamook Cheese? Yum. I am so thankful Sams carries it here in Texas. No other is better. Give it a try too.
I grew up there and will always have my heart in Portland.

Have a wonderful time, I am envious!!!

Trin

I would go straight up on 5, then come back 101. That way you can time it better. Don't forget the redwoods on the way back, also in Cresent City, the Charter house for the best fish!!! Just as you leave the city, sits back in the marina.
Be sure and lock your car at all times, and put valuables in the trunk. Don't know about now, but years ago a out of state car automatically got broken into. Watch your speed.
Have fun and enjoy all the sites.
Oh yes, you cannot pump your own gas. State law. This was put in to put people to work when the logging industry went under many years ago.