Año Nuevo State Park is the site of the largest mainland breeding colony in the world for the northern elephant seal. You have to reserve a spot for the guided walking tour and it's only available from Dec. 15 through March 31st. You can learn more by clicking the link below.
From the visitor center it's just a basically flat 3 mile hike round trip to the location to see the seals. There are some picnic tables near the visitor center and they work great for lunch! It is usually overcast and windy during this time of year so bring a wind jacket and scarf!