Be sure to check the calendar --during the off-season
Legoland is not open every day of the week.
There are pros and cons to going spring vs. summer. In summer, you will have longer park hours and all of the shows/movies open. It should also be warm enough to use the water play area in the Pirate Zone. Of course, it will be hotter and there will be more people. In spring, the hours may be as little as 10 a.m.-5 p.m., many of the shows/3D movies many not be running, but there will be fewer people and low wait times.
As for a beach day, be prepared that the California coast often has morning fog that burns off later. Don't judge a day by how it looks when you first wake up. The water will be cold year-round.
If I were doing another San Diego trip, I'd try to go in the off-season (we went last October). Lodging is cheaper and you can rent a condo for a few nights instead of a week minimum. Crowds are lower at all the attractions and the beach.
PHXscuba