Oneworld alliance airlines are tossing about ridiculously low fares to California. One offer today is Geneva to San Francisco at $252 to $303 round trip for travel dates in April, May, early June, then late August into October 2019. Lowest ticket prices are available through American Airlines.
Geneva GVA – Helsinki HEL – San Francisco SFO – Los Angeles LAX – London LHR – Geneva GVA
Monday Sep 16 – Sunday Oct 13, 2019
I chose long dates in this sample itinerary as the type of ticket I would consider if I wanted to go to Europe twice in four weeks, but there are plenty of one week options too. One way flights in September and October are generally low priced with airlines like Norwegian.