Amusement parks:
Sea World
San Diego Zoo
Wild Animal Park
Legoland

Shopping:
Fashion Valley
Horton Plaza
Plaza de Las America
Carlsbad Outlets

Dining:
Rei Do Gado Brazilian Curuzcaria (in Gaslamp!)
il Fornaio (Italian restaurant in Coronando)
George’s By The Cove (La Jolla)
The Prado (Balboa Park)
Boat House (Shelter Island)

One day each spot. The zoo may be the best one in the world, Tijuana caters to tourists; so there are lots of tourist things to do there and get a flavor for being in a foreign country, and Sea World has shows and sights, including dinner with Shamu the whale, that take all day to see.

You can get to Tijuana by light rail, which can be caught not far from Gas Lamp. You can get to the zoo by bus. I have to believe you can get to Sea World that way, too, but I’ve never done it…always drove.

If you have a car, in the evening, after spending all day at each spot, drive down to Imperial Beach; then up the Silver Strand to have dinner at the Hotel Del Coronado. The ocean and beach views are wonderful.

Go to Old Town by car or light rail and have Mexican food another evening. There are also old-west buildings and places to see there.

On your third evening, walk from Gas Lamp to Anthony’s on the wharf along San Diego Bay. They have the best seafood in the area and great bay views to boot.

http://www.splendad.com/

Buy the San Diego Card and use everything on it All the big and little amusements are included in the price. Sea world, zoo, tour buses, resteruants, museums, etc…
Try http://www.cheap-fun-family-travel.com/California.html for more ideas

As a traveler who strives to use public transportation whenever feasible, I’ve always used it when visiting places like London, Chicago, Rome, Paris, Madrid, etc., because of the comprehensive public transportation networks.

However much of what there is to see in San Diego is not on the Trolley line nor is it well-served by buses. Without a car, you cannot go out to the end of Point Loma, to the top of La Jolla’s Mount Soledad, to Torrey Pines, North County beaches, etc. Yes, you can take the bus to Mission Beach, but it takes quite a while.

So, rather than do without a car, I suggest looking for ways to reduce the cost of renting one.

Prices fluctuate for what seem to be unknown reasons. I check sites often and look for deals. I just checked Yahoo travel for a week in October, and you can get a small car for $110/week, which includes all the taxes and fees.

I have also done this, not only in San Diego but also Portland, Seattle and Chicago [for a trip way out of the city]: pick up the car from a rental office not affiliated with the airport. In these cases, it meant taking public transport to downtown, and getting the rental car there. [there was no extra charge for dropping the car off at the airport...the fees only apply to cars picked up at the airport]

On a trip last year we needed to rent a car [I'm from there so it's usually not necessary], and found a way that saved us a lot:

1
took Bus 992 [http://www.sdmts.com/RouteFiles/routes/pdf/992.pdf] from the airport to downtwon [$2.25, takes 15mins, runs every 15 mins]
2
Walked 3 blocks from the bus stop to a rental car office and saved $80 compared to renting the same car at the airport [$85 actually...I subtracted the $4.50 for the bus ride].

The cost savings isn’t always so great [I wouldn't do it to save $15, but for $80 heck yes], but at least you can check the internet to see if there are price differences. And with mapping sites like mapquest, you can find out where a rental car office is located.

Other reasons for having a car in San Diego:
1
You can stay in lodgings that are a little out of the way [non-tourist areas] that are less expensive.
2
You can cut down on restaurant costs by buying food in markets, carrying it in an inexpensive ice chest [sold at every supermarket] with you in the car’s trunk/boot [get the ice at your hotel..just don't be a hog]. That way, you can have great picnic meals in that great San Diego weather wherever you go….beach, park, etc. [You can leave that ice chest in your hotel room when you check out, and someone will make use of it.] I wouldn’t want you to eliminate eating out in San Diego, but it is a way to cut costs…saving the money for the more memorable meals or activities.

Have a great time in San Diego!