Just reread OP. MidMarch you'll be in great shape. I'd recommend the Big Sur Inn in Big Sur.

Keep in mind that the drive to Key West may not be so pleasant this year, after the 2017 hurricane went thru - debris piled along the roads, I've heard, with buildings still damaged.
Some of Arizona's attractions may be of interest to you:

1)Stay in Sedona (take "rock-climbing" Pink Jeep Tour into the desert to see ancient cliff-dwelling sites; several to choose from) - shop/dine at several different locations around town. Take Pink Jeep Grand Canyon Day-Tour or drive yourself to:

2) South Rim/Grand Canyon (ride the mules down the trail into Interior or take dramatic helicopter tour)- have lunch at one of the Lodges. Or check into rafting opportunities on the river.

3) Scottsdale (Visit Diamondbacks/Rockies Baseball Spring Games, visit local Cactus/Butterfly Park Attraction with dozens of different types of cactus/nice walking trail, dine at "The Mission" restaurant in "Old" Scottsdale; has wonderful guacamole made to order at your table.)

You could either fly into Sky Harbor/Phoenix or Las Vegas (fine dining/see a show) depending on your choice of driving time or activities. Other things to see or do are a Frank Lloyd Wright home in Phoenix, Meteor Crater site in area near Winslow, AZ, etc.

Be advised Nascar races are held in early March in both Phoenix/LV - affect lodging choices.

I like the Arizona idea, too. OP, have you decided?

It is not shorter to fly to Europe than the West Coast from DC. I live in that area and fly to both regularly. DC to London is 7 hrs going and over 8 hrs returning, and that's one of the closest destinations. DC to SFO is only about 6 hrs going and 5 hrs returning.

Death Valley has always been one of our go-to spots for a March vacation. I'd call to get details about the disruption to the Ranch. Construction could definitely ruin the quiet at the oasis, and you definitely would want to have access to the restaurants. Perhaps try Death Valley next year?

Grand Canyon South Rim is pretty, well, grand. It will be cold, though, and most likely have snow. Likewise, Sedona could get snowed in. Just be aware.

Zion National Park and Bryce are beautiful parks. We've been in late March and early April. Somehow we've always missed a snowfall but had warm days mixed with some rainstorms. It's a beautiful place all year round. Fly into Las Vegas, rent a car, and drive 2 1/2 hrs.

If it helps with your decision, we visited the Grand Canyon/Sedona in the timespan of March 5th/7th, and had "light jacket", sunny weather with no snow. Phoenix had much warmer, "t-shirt" weather. Friends went last year & had the same. I'm sure it can change, as with any area's weather; just go for it & check forecasts periodically.

March is a great time to visit any number of places in Southern California. This year seems to be shaping up as a dry winter, unfortunate in general but nice for visitors. Santa Barbara, the desert communities (Polo in Indio, https://empirepolo.com/for-spectators/season-schedule/ )
Also Orange & San Diego County beach communities, even when it isn't quite beach weather.

I agree with Janis suggestions.

March is the best month of the year to visit Phoenix and Tucson areas and the dessert is in full bloom. Many great restaurants to choose from as well as everything else on your wish list. Musical instrument Museum and the Heard Museum are a couple unique options in Phoenix. Horseback riding, kayaking, boating and rafting are all available in the Phoenix area.