Flight
San Fransisco has a large airport, which is 45mins-1hour from Walnut Creek. You can use the BART train to get to Walnut Creek from the airport. It takes roughly 1 hour and costs ~$10 one way
Flight
Oakland has a nice small airport. You can get to Walnut Creek within 30 minutes. There are many Southwest flights in and out of OAK.
Train
You can use the BART to get from SFO to Walnut Creek Area. We would suggest that you use the Pleasant Hill stop on the yellow line, as there are multiple hotels right by this stop. Uber/Lyft other rideshare options are available to get around Walnut Creek/Pleasant Hill area and there is plenty to do.
Hotel