Sea World does not need very extensive introduction. The most famous attraction is Shamu Show, with killer whales splashing willing tourists. Secondary shows include dolphin and beluga whale shows, as well as seal, otter and sea lion show. There are also several aquarium exhibits worth visiting, including Dolphin Pool, where you can feed dolphins the dollar bills (or equivalent in fish :-), and stingray lagoon, where you can pet these surprisingly soft and gentle creatures.
Check show times before heading out to the park to make sure you do not miss start of the show, especially the Shamu show. "Secret" tip for Shamu stadium - if you go down below to the viewing area (the glass wall before the entrance) during the show, you will most liekyl have the place all to yourself, with some great opportunity to interact with the killer whales one on one. Get your Sea World tickets ahead of time on the web, one year pass for the price of daily ticket is quite worth, especially if you are going to visit place more than once (which most likely you will if you have time). Parking is extra $15, factor it into your budget.
Balboa Park & San Diego ZOO
Balboa Park sits just northest of downtown SanDiego, and quite a contrast from bustling city. The park is home to a number of museums, two castles (Balboa and Prado), and famous San Diego Zoo. Allocate at least 2 hours for a walk through the park, and another 3-4 hours for the ZOO.