San Diego in April is a nice time to visit, although it may be chilly at night with a stray rain shower or two. Some buildings and areas are only "lit up" during summer months, but there is still plenty to photograph and you will miss the summer tourists (always a good thing).
Some ideas for night photography (not mentioned by others):
Hotel del Coronado - beautiful when lit at night
Balboa Park - stroll around, many nice buildings, the lily pond with the lighted buildings in the background is picturesque
Downtown: Convention Center, the SD Central Library, walk around and try crossing a bridge to get some height.
Gas Lamp Quarter downtown is a lot of fun, not sure how easy to get photos
Horton Plaza (also downtown) has interesting photo opportunities (be careful about parking in the parking structure unless you dine or buy something there, can be quite pricey to park otherwise!)
Waterfront: Star of India, other ships and buildings, San Diego Admin building (with the art decco touches may actually be better during day)
Some of the piers may be lighted at night, I don't remember
Coronado Bridge - reflected in the Bay waters is nice - if you cross the bridge to Coronado, the city skyline looking back across the Bay is beautiful
If you are willing to drive, the UCSD Library is very unique (see attachment, not my photo)
Of course there are more, but this should keep you busy