Laurent - my recommendation would be to try to focus in one area also... Depending on what you like to ride, and what you WANT to ride that might be different from what you ride in France, maybe? April is too early for the higher elevation stuff, but there is still some fun lower-elevation stuff! Annadel and Boggs, both a couple hours North of San Francisco are high on my list. For the spectacle, or if you want to race, the Sea Otter Classic near Monterey (a couple hours drive South of SF) is in April. Henry Coe is an hour or so East from there, and some of the riding in Auburn should be good around that time ~3 hours away. Bidwell park in Chico is about 2 hours from there... In Southern Cal, the mountains east of San Diego also have some good riding, and the Anza Borrego Desert east of there has some fun desert riding too. Don't know if I'd drive that far for it though - I lived in San Diego for 37 years and don't think I'd ever move back. Rocky, technical riding abounds though, if you want to get some of that in! While some great areas in Nor Cal are a few hours apart at most - the drive to SD is all day, and you have to get through Los Angeles. Good riding in the mountains east of LA, but it might be early and there were huge fires there a few years ago -
If I were making the trip, I'd probably hed South an hour or so, hit the UCSC trails/Wilder Ranch, then down to Fort Ord (Sea Otter location) for a day, East to Henry Coe for a couple of days, then up towards Auburn (East of Sacramento) and then up to Bidwell in Chico. Head Southwest to Boggs Mtn, and then to Santa Rosa to Annadel. Maybe ride in Marin just to say you did the fireroads on Mt Tam and you are back in SF... Have fun whatever you do!