Hayward, CA to Lynnwood, WA

Straight up the I-5. North of Sacramento it is a lovely drive. There are mountains around Shasta, really, but it is freeway all the way so you really don't need to concern yourself with going slow and driving careful. People will easily pass you. A few short sections were it may drop to one lane each way, but still an easily drive. Also mountains in Oregon north of Ashford. Still, freeway driving. Plenty of places to stay at RedBluff, Ca. Then a couple of nice campgrounds at Weed. Seven Feathers RV park Casino in Oregon is 5 star. Fuel stops on the 5 are frequent. I enjoy the drive.

They are in the SF area so no Grapevine to worry about. Siskayou is not a difficult climb as long as the tow vehicle is adequate for the trailer being pulled.
Neither I-80 or I-5 are particularly inspiring scenery wise except for the Mt Shasta area but that is the easiest route. Coming home treat yourselves by leaving I-5 at Olympia or Vancouver for Astoria then 101 south, it is one of the most scenic drives in the country.

101, several places in OR/WA to pickup 5 w/o mountains.

But 5 is a good road to go over the Siskiyous and it's not like 11,000' CO mountains.

How much time do you have?

You could drive up 101 most of the way and then cut over to I-5. Oregon Coast is very scenic.

donn...The only "Grapevine" I know is in southern CA.

I5 straight shot. Only real mountains you will encounter are the Grapevine and siskiyou summit.
97 north from Weed scenic. Miss siskiyou but will have to down and up at the Columbia river and north of Goldendale
395 scenic. Easy drive most of the way. Some mountains.