First priority should be the loop drive up and over Mt. Hood, stopping at Timberline lodge which is at an elevation of about 5,500 ft. The mountain itself goes to 11,000 ft. Great views from the lodge and the lodge itself is great to see. Then down Hwy 35 thru Hood River, (stopping on the way at some of the fruit stands--it's quite a fruit growing area) to I-84. Take your choice at that point, going east to see the desert (not very far), or west thru the Gorge, which you will go thru to get back to Portland even if you divert east to the desert. The Gorge is the only low elevation cut thru the Cascade Mountains and it's dry on the east side and wet on the west. Several waterfalls, but the must see is Multnomah Falls. Lock your car and don't leave valuables in sight. Several trails, some easy, some steep. Watch out for poison oak, "leaves of 3, let it be"; it will have a lot of red on the leaves in October. This loop is about a half day trip if you just drive straight thru with minimal stopping.
If you are into wine, there are lots of wineries on the west side of the Willamete Valley, south of Portland.
The coast: Follow the Columbia River out Hwy 30 to Astoria, then maybe cross the river to Long Beach, Washington, a touristy destination. Kite flying is big in that area.
Down the coast is very touristy, and there are several highways back over the Coast Range so you can make the trip as long or as short as you want (well, several hours minimum for the shortest option). Central Oregon coast is dramatic with steep cliffs and smashing waves on the rocks below. Several viewpoints.
Portland is a nice city to visit (I wouldn't want to live there) and one famous tourist stop is a bookstore: Powell's Bookstore. Covers 2 or 3 city blocks. New & used books.
Now I'm running out of ideas, but I've just scratched the surface.
Early October usually has very nice weather, 3rd weekend it chancey, so if you have the option, take your extra time early rather than later.
