If your coastal California road trip includes San Diego, try tide pooling. Here's how: A list of the best tide pools in San Diego, a fun free thing to do outside with kids and without in winter.
7 Family-friendly tide pools in Orange County, California.
Tide Pool - Hulopoe Bay, Hawaii, As a child I would search for shells and sea urchins with my sister and Mother@ Caloundra Qld just like this. It brings back very distant but wonderful memories.
For a scenic day trip you must take some time to explore Cabrillo National Park in Point Loma. Go for the history, but stick around for the marvelous trails and tide pools.