if you have a weekend
If you have just 2 days – go to La Jolla. You can spend time at the La Jolla cove, take in the coastline, then go to eat/shop. Apart from wonderful beaches, the San Diego Zoo is also located nearby so this is perfect for people who have less time.
if you have less than a week
If you have more time, i recommend starting with La Jolla but then you could choose between Old Town and Little Italy. Old town is very touristy, so if you’re not into that – skip. If you have closer to a week, then i recommend spending 1 day at either Disneyland, universal studios or Legoland – choose whichever interests you most, though Universal studios is in LA and the other 2 are in Orange county. All are a bit of a drive, with LA being the farthest – this is why i recommend picking up one of them. If you’re really into amusement parks and nothing else – go for all 3!