My German friends from Liepzig visited us last spring (we switch off visiting every couple years, our turn to GE next). They originally wanted to do AZ pretty much. they are non-bikers though
I spent 6 yrs in HB in socal working and knew they should see some of CA.
We left phoenix, went to HB for a night, up-to Santa Monica/LA and took the tour bus thingy to hollywood, etc.
went to Morro Bay for the day/night, that area is so cool...
then up to highway 1, we drove all the way to the spot where they had another landslide (toward the south end from big sur), so no farther without going around.
then across CA to the grand canyon and back to phoenix, 2500 miles on a rental van in about 7 days I think it was. We didn't have time to go by the big trees, which I really wanted to. Next time they come we will go there first.
I was coming back from a biz trip once, west on 70, into Utah, got up on the last day of May in the morn and headed up the mountain to get over the continental divide, about 8-9K feet I believe. Yikes, I hit fresh snow on the road (2-4" deep), and rode through it for 25 miles before I got down the mountain. The lady down the road said there was a freak snowstorm that night, some harley guy got caught up there and had to get a tow off the mountain.
just sayin, I would have never expected snow at that time of the year or I wouldn't have gone that way!!
I think I've seen ice on the road in CO up near Estes Park in June too but you can usually ride up aways to see if/when you have to turn back. Ya'll will have a blast either way.