UC Davis is approximately 70 miles east of San Francisco and 15 miles west of Sacramento along interstate I-80. Follow signs to "UC Davis" exit. Follow the exit road to north of the freeway via "Old Davis Road." You can find this road on the campus map linked to the main UC Davis home page. (The link is located near the top of the home page.) The Physics Department is in the Physics and Geology Building with coordinates as listed in the map index.
Parking information can be found under the "Visiting" link near the top of the UC Davis home page. Parking details are under the link on the "Visiting" page listed as "Plan your visit". Also, on the road entering campus you will see a visitor booth. You can ask questions there on how to park and get to the Physics Department.
Sacramento airport is the most convenient airport to fly into when visiting Davis. A short shuttle ride takes you to campus. In most circumstances, there is no need for a car since the hotels and downtown are within easy walking distance of the department.
Travelling back and forth between UC Davis and UC Berkeley is made easy by the Davis/Berkeley Intercampus Shuttle. You will need to search for this under google. The web link changes sufficiently frequently that it is not useful to specify it here.