Certainly don't miss the redwoods. However, a few words of advice: don't believe the map when you are planning drive times back out of Redwoods National Park. Hwy 299 back out of the park to Redding will take at least 4 hours, more if the road is under construction (again and always, it seems). It's a twisty-turny slow crawl at the best of times. Hwy 101 to Hwy 199 is not much better; the slog into Grants Pass in Oregon (where you meet up with I-5) will take time in the summer due to all the tourist-RV traffic. Staying on 101 will be slow at best. The views are worth it, but it will not be a one-day trip! Realize you will spend more than a day doing it, and all will be well (sightseeing and driving, that is). If you are pressed for time, a stop in the Muirs will be better time-wise, although not as majestic. If you've never been to Point Reyes, a stop is recommended.
You might see signs pointing to the
Sea Lion Caves on the Oregon side of 101. I do NOT recommend doing it in the summer! The smell is astounding, and I always tell people to do it in the dead of winter ONLY. If you still want to go, arrive early... There's a side benefit of going that way, which is the Oregon Sand Dune Recreation Area. Pack lots of water, bring your own shade, wear a hat, and have fun!
Booklovers will not want to miss
Powell's Books in Portland. Several locations! Call ahead and have them hold books for you. Another good thing: they take AMEX, so you can justify the books as adding up to a free ticket.

Or you could try Crater Lake National Park, in Oregon, or Mt. Rainier in Washington, or... how long are you going to be here? Anyway, no V-flyer should miss the
Museum of Flight on I-5 (Seattle Area) or a trip to Everett to take the assembly-line tour of Boeing's plant at Paine Field. You could also
Ride the Ducks!
Lots to do up here! (Better stop now, I'll go on all day...) If GJ sees this, I'm sure he'll add his own two cents' worth.
(...editing highway numbers after checking the map...)