Let's say you fly in to Portland and drive south to Redwoods...

I'd drive to Cannon Beach from Portland and then head south on the coast route. But I'm an old surf dude so I have a real affection for the Pacific Ocean.
 
If you're taking I-5 south then Mount Shasta is a nice stop.
 
Regards,

Where is crater lake?

Sounds like a great trip. Particularly if you visit Mendocino.

As you go east from Portland the Gorge (as it's called) is really stunning.  There's a very distinct change when it goes from the rainy area to desert area too.

Hi,
 
2 ways to think about it.
 
Fast and Slow!
 
Fast. Straight down I-5, and then take 199 in Grants Pass. Also a nice drive and it gets beautiful green just south of Eugene, all the way to Crescent City. You can stop in Grants Pass for refreshments, and then an hour plus to Crescent City, and the Redwoods.
 
My favorite place in the world!
 
Slow. Drive Highway 126 all the way to the coast and then go down the coast all the way to Crescent City on Highway 101. Very beautiful drive and you will want to stop and take pictures, specially if you are on a hot date!
 
If you fly into Portland, you should also plan on spending a night somewhere and then call me, or Craig (if he's being hospitable) and we can have a nice evening doing nothing and more nothing and sweet buck'all nothing, and just enjoy a good yap ... I might even take you guys to the Brazilian Restaurant!
 
Portland is nice, but it is a really big small town and its tastes are almost exactly like a small towns.

Have lived on the west coast all my life...
 
As has been mentioned, head east along the scenic route for the Columbia River Gorge (you exit from the freeway as you leave Portland).  There are several fabulous waterfalls along that route, Horsetail and the well known Multnomah Falls, to name two.  Crown Point has a nice vista of much of the gorge.
 
Astoria is good, or take just about any of the two lane roads over from the valley to the coast.  A stop at Gold Beach is another option, the sand dunes at Florence might be of interest, and Depoe Bay is a very tiny and quaint place.  On the way back, if you take the route from the coast back to Grants Pass, a stop at the Oregon Caves could be fun (one of the larger cave networks outside those at Carlsbad, NM.  Overall, IMO, US 101 is far more scenic that I5.  If you like Victorian architecture, the Carson Mansion in Eureka is worth a look.  Jedediah Smith Redwoods and Redwoods National Park, as others have mentioned, are almost "must" stops.  If you like more-or-less dirt roads, consider taking a detour to fern canyon on the coast.  Also if it still exists, Lazio's in Eureka has very good seafood.  Crater Lake is futher east than it sounds like you might be going.  Bend, on the east side of the Cascades is a jumping off point for much of the scenery in eastern Oregon.  If you like wading in the ocean, the beach at Seaside is unusual for a west coast beach in that you can wade quite a ways out into the ocean.  Note that Seaside and Cannon Beach are popular tourist places.
 
As for "rain forest" you would need to head north up to the Olympic Peninsula.  The Olympic Peninsula is unique in that it contains a temperate rain forest.  The Hoh River Valley averages something like 175 inches of rain a year.  IIRC, the only town in the area is Forks (sorry, no town named Knives or Spoons).
 
Anyway, have fun.