On the south side of Cardiff-by-the-Sea between the 5 Freeway and Highway 101 is San Elijo Lagoon Nature Center.
Along the walk you might get lucky and spot silver striped mullet (that I've talked about before) jumping out of the water, spot a snake, or even see a butterfly caught in a web.
They have wonderful events at the nature center too. On one of our visits we arrived just in time for the nature center's regular story time. My kids can read children's books, but they still love to have books read to them, and the books read during story time were all nature inspired.
During another visit they had a special bird event, where rangers spoke to the kids about the different types of birds in the area, the kids made a bird marionette and they did various activities to learn how birds pick up food and what makes each type of bird different.
Keep up-to-date with other events at the San Elijo Lagoon Nature Center on their website.