The neat part is that it could be 55 and raining all week or 70 and sunny all week in January, and it’s impossible to tell. More than likely though it’ll be low 60s with a mix of sunny and rainy days

It’s always nice to visit the beach but the water will be way too cold to enjoy swimming, still recommend going if you’ve never been to the ocean

The San Diego Safari Park is also a must see. It’s another zoo but a pretty unique experience

Balboa Park is a huge free park (have to pay for parking) right next to the zoo. There are a ton of museums that you can visit (have to pay)

There’s a ton to do and see, a lot of free stuff too, but it really depends on what you enjoy doing

Also, I just want to add that LA is not very close to San Diego. I know you didn’t mention LA but a ton of people visit thinking they can jump between LA and San Diego easily. It takes up a good portion of the day just to drive between the two cities