Balboa park is great, if they haven't changed things they used to have free admission to the museums at certain times during the week, it changed every day. The park is bigger than central park in new york and also where the zoo is located.
Point loma is one of my favorite places, go at low tide and you can visit the tide pools as well as check out the light house and awesome view.
It's pricey but even if you can't afford it do stop by the hotel del corondado, Coronado has a lot of neat little shops to visit as well.
I'd suggest the seal tour, I'm sure you can find info online or once you get to a hotel there, it's a land and water tour and we got to get real close to a bunch of sea lions in the ocean.
sea port village is really neat, I think that's where we found the seal tour
Also make sure to check out all the naval ships and such, although I don't recall where they are located.
there is an indian reservation in the area, that has a great outlet mall and a water and light show after dark, it's in the mountains, so take a sweater it will get chilly after dark.
while it's normally sunny and warm, march and april could be rainy, and it does get chilly at night. have fun!