I lived in the San Jose area for many years.
April in the valley is mild. Depending on year, some rain. But most of the rain comes in March. Temps in the 60s to 70s(f). 50s(f) at night.
San Francisco, though it was only an hr away from us, was almost always 5 to 10(f) cooler.
Expect 50s or low 60s for most part in and around San Francisco.
And as you cross the valley (Stockton, Modesto) it will be 5 to 10(f) hotter.
But once you reach the Sierras, things change fast. Up high, it can still be pretty cold sometimes during the day, but usually 50s in April, but sometimes hotter, into the 60s. At night it can get cold.
I'd say, be prepared for mostly 50s-range temps, but don't be surprised if it gets near freezing at times or at night. But then, it might be in the 60s the whole time! You really never know.
But in a nutshell:
1) It's definitely doable.
2) The park is nice almost always, but early spring can be beautiful.
3) See above.
4) Be prepared for some cold weather riding but layer up, because it can change real fast.
Have fun, man.
Ride safe, Gary
2018 Honda Africa Twin Adventure Sports - Manual.
Setup: Givi Tall Sport Touring Windscreen; Hepco & Becker Lower Engine Guards; Hepco & Becker Handle Bar/ Hand Guard Protectors; Hepco & Becker Tank Guard Soft Bags for OEM Uppers; HTTMT Kickstand Pad Extender (Red);
RAM Mounts X-Grip; Swegotech Forkshield; Touratech Zega Mundo Panniers and 2018 Rack (31/38L); Touratech 20mm Bridged Handlebar Riser; OEM center stand; OEM skid-plate, uppers and tires.