Wow, how can you choose?
So...you want to take an afternoon spin.
First of all, make sure you bring weather gear. It's likely to be cool (40's or 50's) and damp (not the outright RAIN we had in the south, but you'll be wet if you don't have good outerwear).
You're going to want to line up a bike in advance. REI and Gateway Bicycles are the two places I would think of.
Assuming you have a rental car to pick a starting point, I would be remiss in not suggesting a Gorge ride: start in Troutdale near Lewis & Clark State Park, and then climb all the way to Crown Point. Check out the restored museum there, and then turn around and take a free ride the way you came.
If you had just a little more time, I would strongly urge you to ride down the other side from Crown Point down to Multnomah Falls. It's the highest falls in North America, and there are always (literal) bus loads of Asian tourists taking pictures of it. The climb back to Crown Point is designed for Model A Ford's, so it's quite a bit of work but never too steep.
If you think Troutdale is a bit far, you could shorten the ride by starting at Portland Women's Forum, or even start at the Falls and work westward back to Crown Point.
There are some closer-in rides I could suggest, but they pale in comparison, in terms of raw jaw-dropping scenery.
I'm not sure I could join you, though I'm tempted. We'd have to see just how busy my job is that day--but that's another advantage of the Scenic Highway ride: navigation is not a big deal.
Good luck, and let us know what you think of the Pacific North*WET*!
ORS 811.065 (1)(a)
The driver of a motor vehicle may only pass a person operating a bicycle by driving to the left of the bicycle at a safe distance and returning to the lane of travel once the motor vehicle is safely clear of the overtaken bicycle. For the purposes of this paragraph, a “safe distance” means a distance that is sufficient to prevent contact with the person operating the bicycle if the person were to fall into the driver’s lane of traffic....
LCI #2105 Lambchop Rides!