Balboa Park
Balboa Park is an enormous park right in the center of San Diego. Fun fact- it is actually larger than Central Park in NYC! It is probably one of the most beautiful parks I have ever seen. There is just so much to see and do here and there is so much you might miss, make sure you have a full day to take it all in. I recommend taking a car to the main parking entrance to get started.
Places to check out:
The Spreckels Organ Pavillion
The Botanic Garden and Lily Pond ( the indoors is closed right now)
Japanese Garden
The Sculpture Garden
Spanish Village Arts Center
Palm Canyon
California Tower
Trees for Health Medicinal Plant Garden
The Cactus Garden
The Museums ( they are mostly closed right now but will open soon)
San Diego Zoo ( although I recommend making this a separate day trip since it is massive and will take up the entire day)
After you are done I recommend walking down the Cabrillo bridge where you can see views of San Diego. Then have lunch at one of the yummy restaurants in Bankers Hill.