Couple ways to extend the route if you're interested.
The little bit I've been through of Northeastern Nevada...wasn't the best time I've had on the road. It'd be a bit of a trip, and it's not much better driving, but were you to head north on 84/15 and drive through southern Idaho, you could hit Atomic City (first place powered by nukes) and EBR-1
- first commercial reactor. They've got a nice facility there to see some history of nuclear power. Craters of the moon park is nearby, NASA tested (still does?) lunar rovers there. Newer rovers have also been tested outside Moses Lake WA, you'll pass by that it appears. Further west in Boise you'd be able to hit the Birds of Prey center
- pretty neat for all ages. There area few other things to see in the Boise area if you're interested as well.
Two things you could add to the coastal run are Monterey to the south - Gorgeous city, well worth the loop and Seattle to the north. The aquarium in Monterey is one of the best I've seen(particularly the Lion's Mane Jelly tank). Seattle of course has more to do than one post would do justice.
Not sure the reason for a stop in St Maries, but as a route there the Palouse highway is effective, but that's about it. I'd just as soon for a road trip go through Coeur d'Alene - a bit to do there if you'd like, and run down Hwy 97 on the east side of the lake through Harrison. Beautiful drive, though my wife needs motion sickness pills for it. No worse than driving through the redwoods of California though. On the way east Wallace has some neat mining museums, the tours are well worth it.
I've driven the coastal route and then some several times, never gets old. Worked in Boise one summer, and live in Spokane - give a shout when you're through.