Tillamook Forest Center is worth a stop if you're on Highway 6. Ditto the Tillamook Air Museum, if only to gawk at the size of the building.
I like the Leach Botanical Garden, but I'm not sure how much it would appeal to a teenager. It's probably a bit out of the way, too. If the weather's clear, though, Powell Butte has some good hiking and views.
If you head out the Gorge way, I recommend the Gorge Interpretive Center in Stevenson, WA; the Hood River fruit loop; and the Western Antique Air and Automobile Museum in Hood River. (Especially the fruit loop, since we're heading into apple season.) One of my favorite drives is to do a loop out highway 26 to the Hood River Valley and come back on I-84. Lets you hit Timberline Lodge, too.