Taking the four kids (ages 9-18) to San Diego to visit family. What are the "must sees" for us while we are there? None of us have been there before.

Hard to know what you'd like without a little more info; time of year, budget, length
of stay, etc.

But, These are my favorites:
The zoo and Balboa Park museums.
Ride the carousel by the zoo parking and try for the gold ring.
Old Town
Hotel del Coronado. Rent bikes and cruise around the island.
Mission Beach/Belmont park
Harbor cruise (narrated tour) or ride on the coronado ferry (much cheaper)
Wild Animal Park (whatever the new name is...)
Cabrillo Lighthouse
La Jolla
Star of India

aircraft carrier Midway

The Birch Aquarium is very interesting and would be fun for kids. They have ponds in the back where you can touch the starfish, sea cucumbers and anemones.

Tour of the Aircraft carrier Enterprise was very interesting. If you are going in the late Spring or Summer there is always the Padres games and I agree about the zoo - great. Also the younger one might enjoy Lego Land and Sea World is a good option as well for the whole family.

The Zoo for sure -- one of the most amazing zoos in the world. Seaworld is also great.

If in summer, I'd definitely head over for a day in PB -- Pacific Beach. Maybe take a surf lesson. Great for families.