Some good resturants at reasonable prices -
Sam Choys Bali Hai on Shelter Island -
http://www.samchoysbalihai.com/ - great views across SD Bay to downtown - hawaiian style menu -
http://sandiego.citysearch.com/profile/246670
Old Town Mexican Cafe - In Old Town San Diego, great food, good price - gets crowded every night of the week - if there is a long wait and room at the bar - head to the bar, you can order all the same food there.
http://sandiego.citysearch.com/profile/86401
Taste of Thai - Hillcrest area, for Thai food.
http://sandiego.citysearch.com/profile/91075/
Extraordinary Desserts - Just north of downtown off the beaten path but well worth the trip.
http://sandiego.citysearch.com/profile/87100
The Prado Restaurant - in Balboa Park, if you head to the park for a quick walk around, make some time to eat here.
http://sandiego.citysearch.com/profile/167743
Lots of resturants, live music, and dance clubs - Gaslamp District Downtown.
As for diving, something you probably really wanted to learn about right? If there is a boat heading for San Clemente Island for the day (it's about a 70 mile boat ride from SD) - that would be the best diving you could find in this area imho. Otherwise you can go off the beach at La Jolla Cove, Marine Room area just south of La Jolla shores. Local boats will head to the Yukon, Ruby E. Point Loma Kelp Beds (20 minutes ride) & Coronado Islands (1-1.5 hours ride) - here is a list of dive boats in SD -
http://www.californiadiveboats.com/
Other thiings - Scripts Aquarrium as mentioned is very nice. The SD Zoo is also a wonderful place. Unless you want to see the big fish - I would not bother with Sea World.
have a great trip!