Road trip from Seattle to L.A. - suggested route and places of interest?

Welcome.

We're from Melbourne, and have done similar trips both ways, over the years ... it's wonderful.

I would budget the whole three weeks if you can fit it in ... there's a lot to see and do. In relation to your proposed itinerary points, it's mostly very good, however I would offer these comments:

• think of your steps in terms of nights rather than days - certainly clearer
• after Portland (three nights is plenty), travel the Columbia River Gorge to Hood River
• visit Multnomah Falls and other very impressive sites
• visit Mt Hood (as impressive as Mt Rainier)
• travel via Bend to Crater Lake NP (outstanding), sleep at Medford or Grants Pass
• disclosure: we were not big fans of the Oregon Coast
• Oregon Caves National Monument I don't know, but it's right on the way
• visit Jedediah Smith Redwoods State Park before you get int Crescent City
• visit Del Norte SP, Prairie Creek SP, and Redwoods NP
• call into the museum at the Tree of Mystery roadside stop in Klamath
• turn inland at Arcata, and go via Redding to Lassen Volcanic NP
• then via Truckee, Lake Tahoe, and Yosemite NP from the east (Hwy 395)
• three nights at Yosemite NP is good (check the Bug Hostel - great rooms)
• then Sacramento > San Francisco (3-4 nights) > Monterey
• need accomm with parking in San Francisco
• at Monterey visit Point Lobos Nature Reserve and Carmel-by-the-Sea
• your child might like the famous Aquarium in Monterey
• two nights on the road from Monterey to LA (probably Cambria and Santa Barbara)

I trust some of that is useful - it's based on pretty solid experience in both May and September / October.

We use lower-end Wyndham Group motels (Super 8, Days Inn, etc - they're fine - you get more dollar-for-dollar than you do from a motel in Australia; they often include a hi-carb breakfast).

Worth buying a National Park Annual Pass at your first park (Mt Rainier probably) - $US80.00.

Sacramento is a big detour (even if you use the route suggested above) and you will probably enjoy spending more time elsewhere. There is stuff to do there, but other places would be better. Go only if you have friends or family you want to see.

The road down the coast south is currently closed south of Big Sur due to a mudslide. The target for re-opening is mid-September, but that can mean anything. The next update will be in July. This means that if you want to see Big Sur, you will have to drive south from Monterey, then double back & and continue on to LA via an inland route.

Be sure to book lodging in Yosemite as soon as possible. It’s easier to get at that time of year than in summer, but it can book up.

You have picked a good time of year. Mid-September to mid-October is when the coast has its best weather. Warmest (although that is a relative term) and most likely to be fog free. (unless

Yes - to save a night or two you could miss Sacramento, and just head straight to San Francisco from Yosemite NP (on my itinerary suggestion).

We did visit Sacramento on one trip, and enjoyed it, however we had a special purpose in visiting.

Here is a 21-night program that drops Sacramento altogether:

29-Sep - Seattle
30-Sep - Seattle
01-Oct - Seattle
02-Oct - Portland
03-Oct - Portland
04-Oct - Bend
05-Oct - Grants Pass
06-Oct - Crescent City
07-Oct - Arcata
08-Oct - Redding
09-Oct - Chester
10-Oct - Lake Tahoe
11-Oct - Yosemite NP
12-Oct - Yosemite NP
13-Oct - Yosemite NP
14-Oct - San Francisco
15-Oct - San Francisco
16-Oct - Monterey
17-Oct - Cambria
18-Oct - Santa Barbara
19-Oct - LA

It also only gives you just two nights in both Portland and San Francisco, but I don't think that's too big a concession on a parks-focused road-trip. If you can add a few more days, you would be well rewarded, by either more time in the bigger cities, or by reducing couple of the longer drive days embedded above.

Instead of staying in Grants Pass, OR, I would stay in Ashland, OR which far more interesting.

I tend to work on the premise that if you're arriving late and leaving early, it doesn't matter too much, particularly if a few miles can be shaved off. And wasn't Ashland's heyday at least 25 years ago?

I haven't been to Ashland in years, so maybe you are right.

I've heard that it does Shakespeare, but that's hardly relevant to the road-tripper ducking in to overnight at the local Super8, and it had a small role playing itself in Reese Witherspoon's "Wild". A couple of other artsy towns that failed to deliver for us were Floyd VA and Asheville NC.

For similar reasons I've never been tempted to head to Austin, Nashville, or Memphis.

I live in California and write about family road trips in the state. To add to your mix or "fill the gaps" as you say, think about stops in Mendocino County, Sequoia/Kings Canyon National Park, Santa Barbara. I have a lot detailed itineraries on my blog that would answer your needs, here is just an example: http://familyroadtripguru.com/2018/03/20/mendocino-county-family-road-trip-itinerary-part-i/
http://familyroadtripguru.com/2018/04/30/giant-sequoia-and-kings-canyon-national-parks-family-road-trip-itinerary/

Thanks for all your suggestions, all very helpful.