For PDX, you will want to post to pdxkayaker.org yahoo group or the facebook page. Professor paddle is a great resource, but mostly for Washington.
Next Adventure Next Adventure
does free demos of boats. You might want to call them ahead to make sure they are cool with it, but it may save you some money.
There are always runs doing the Upper Clackamas (Class III+ to IV at current flows). It is an hour drive from Portland.
It sounds like you are wanting something a little more advanced, so I would recommend The upper sections of the White Salmon, which is 1.5-2 hours from Portland near Hood River oregon (across the river in Washington). The Green Truss section of the White Salmon is a NW Classic. Experienced TL is required for this run and folks may be a little bit hesitant to take a stranger with unknown skills. I would call some of the outfitters in Husum Washington (Wet Planet, River Drifters etc..) to see if any of their folks are running then. Worst case scenario, you can run the middle white which is a great run. It has been Class IV recently with higher than average water (4.5 feet on the Husum gauge). Husum falls isn't really being run right now - as the boil line is like 25 feet from the base of the rapid...meaning a swim would be....sporty.
Also, Opal Creek is just a bit low right now...if there is significant rain before you get here and the Little North Santiam comes up Opal Creek is one of the most beautiful runs you could paddle. Also about 1.5-2 hours from Portland.