Sounds like you're having a wonderful trip. If you like Death Valley now, you could come back about March or April when the flowers are out. I was there during one Superbloom, but I hit another couple years were the flowers were more subtly beautiful.
It you took Interstate 80 from Sacramento over Donner Pass, you were about 300 yards from my home.
You'll love Grand Canyon. There's something wonderful about finding viewing spots all your own along the canyon rim. Take the shuttle bus and get off for 15 minutes at every stop - each stop is a wonderland. Same thing with the shuttle bus at Zion National Park, if you happen to get there. And if you have to pay a little extra to stay at a lodge inside a National Park, do it. We scored a $91 cabin at Old Faithful Lodge at Yellowstone this year - wow! It was a thrill to see an elk outside my door at 6 AM.
-Joe-