We have traveled to Coronado (with OUR girl) several times in mid to late February and have found the weather lovely, compared to our winters in New England! It has often been in the mid 70's (though colder at night....mid 40's to 50s) The water is nippy, but our hotel had a heated pool and the bravest of us did dip our toes in the beach while collecting shells.
Coronado is very quiet and has a small town feel. The Hotel Del Coronado has many shops (including a toystore and ice cream shop) in the basement and of course one of the best beaches in the country behind it.
You can rent Bikes and even surries with fringed tops at Bikes and Beyond at the Ferry Landing on the harbor side of Coronado and you can ride them down to Tidelands Park where they have picnic facilities and a small sandy playground. The park has a great view of the downtown skyline, the Coronado Bay bridge and you often see huge ships from the Naval station traveling up the harbor.
From the Ferry Landing on Coronado you can also take the ferry to downtown and visit the Star of India Ship or the Seaport Village (mostly shops, but a great antique carousel for the little one) both are walking distance from the ferry.
Balboa Park is our favorite place. They have "free Tuesdays" when an alternating selection of museums are open for free. Our favorite is The Ruben Fleet science center. On Sundays ONLY (during winter), the Carousel in Balboa park and a small train ride are operating and they also have free outdoor concerts at the Spreckels organ pavillion in the park and an afternoon puppet show at the Mary Hitchcock puppet theater in the park for a slight charge. There is also a great playground on the Park St. side of the park for a place for little ones to un-leash some energy.
LaJolla is definitely a must if you want to see seals on the beach at the cove they call "The Children's Pool" (bring a sweater as the viewing area extends out to the sea and it can be quite windy. ) There is also a small, but great, acquarium in LaJolla called the Birch Acquarium that is inexpensive and just the right size for young ones, with a touch tidal pool and, on Tuesdays, they have an event with a man in scuba gear in a feeding tank who can answer kid's questions while the fish and tiger sharks swim around him. There is also some cool interactive stuff for kids.
SeaWorld is of course, SeaWorld.....we took my daughter when she was older and she loved it....but it is huge (like the Zoo) and can be a bit much for the toddler set. When we went last time they were offering a year admission for the same price as the day ticket and though we live so far we took it ,and went twice during our visit (and still didn't see everything!
Belmont Park on the beach at Mission Bay has a few (very old)amusement park rides and shops and arcadesbut the rides only operate on weekends in the Winter. The beach there is huge and has lots of surfers and we enjoy just wandering around there.
LegoLand in Carlsbad also has shorter hours (and long lines) in the Winter but the young ones love it.
Old Town has become very commercial but my little one still loves it especially at night when the lights are on and the shops have all there crafts and pottery and such outdoors, and she loves to see the Mexican ladies making the tortillas in the windows of the restaurants.
Well...I guess I've gone on quite a bit. San Diego is one of our favorite trips! I hope you love it, too.