Barbara,
I grew up in Portland and we now have a vacation home in Gleneden Beach (Lincoln City area) on the coast. I'd recommend the coast over Central Oregon. Don't get me wrong Central Oregon is very pretty and we spent many winters there enjoying the awesome skiing. But, there is nothing like the Oregon coast. I know you don't get in until noon but, to help you with timing, you can make the loop from Portland to The Dalles in one day, although it makes for a long day. If you do that loop make sure to visit the roadside fruit stands in the Hood River Valley. Also, Hood River is the WindSurfing Capital...and a trip to Timberline Lodge on Mount Hood is a must. The lodge was used for the exterior shots of the movie "The Shining." It's a beautiful place both inside and out.
As for the coast, Cannon Beach is very quaint, artsy type community. As you head south on US 101 (great drive by the way). Be sure to stop at the Tillamook Cheese Factory. You can watch cheese being made and even if you don't like cheese, they have ice cream that is some of the best. I only use Tillamook dairy products. YUM!!
Pacific City (a little south of Tillamook) is another nice community. Don't miss the Brew Pub (good food, nice view) located next to Cape Kiwanda. If you're lucky you'll be able see the dory boats launching through the surf on the beach. There's also a sandstone rock you can climb (it looks harder than it is) and the views from the top are tremendous!
Lincoln City is the next town you'll come to. Great beaches and some good restaurants. We like Mos (I've never had a bad meal there in 30 plus years, although some say it's not what it used to be), The Blackfish is good and if you want a nice dinner(but be prepared to pay for it) try the Bay House.
Another favorite restaurant of ours is Tidal Raves in Depoe Bay, which is the little town just south of Lincoln City (about 10 miles south). They have some of the best food on the coast. Depoe Bay is pretty much full of tourist shops and lots of junky stuff.
Next town down is Newport. Must sees here is the Oregon Coast Aquarium. Keiko from the "Free Willy" movies lived here after he was rescued from Mexico. The aquarium is not just for kids, my husband loves it there!
Down around Florence you've got the sand dunes. Great fun.
I used to live in Coos Bay and really can't think of any reason you'd want to go that far south. But that's just my opinion.
You can cut back to I-5 at Florence, which will put you in Eugene and from there you can I-5 south the Crater Lake.
Crater Lake is a MUST see. And as SillyBilly said Silver Creek Falls near Salem are some of the prettiest in the state.
I don't envy you and the choices you have to make. I think all Oregon is beautiful, each area in its own way. Good luck!