Since the finale of the air races is Sat and Sunday Sept 19-20, the same dates as the world-famous Fox 15 Hurl and associated stunt contest near Sacramento, chances are the UK contingent will choose to attend the air races and miss the Hurl. Lousy timing.
What to do and see around Northern California? It depends on how long the group has and what particular things they may already have mapped out. What airport is the group arriving at, on what date, and when do they depart, from what airport?
If it fits into the schedule I am positive the NorCal stunt community can provide an opportunity to test the dry California air to compare it with the soggy stuff you have at home. There are dedicated CL fields at Napa (one hour north of San Francisco) and Alameda, next to Oakland across the bay from San Francisco. Planes etc can be provided.
Depending on time available, some of the most spectacular coastline in the world is found north and south of San Francisco. That Lake Tahoe thing just west of Reno is worth a look, despite Brett's encouragement to skip it in favor of the Hurl. Let us locals know what the itinerary looks like and we will pitch in to make the trip more enjoyable. And our local microbrew beers are the equal of anything anywhere.