I might stop at Antelope Valley California Poppy Reserve SNR soon. It is a little bit off the I-5 route between NorCal and SoCal (near Lancaster and Palmdale).
Recent photos:
http://www.parks.ca.gov/?page_id=24984
Info:
http://www.parks.ca.gov/?page_id=627
Perhaps it's not as spectacular as Anza-Borrego Desert or other spots, but it's more accessible if you are already planning a north/south roadtrip on I-5.