San Diego is a lot of fun and has a lot to do in the area. A rental car would be great to easily get around and so you can go to the beach with ease but beach parking is always a pain so keep that in mind. You can also use the car to take the 2 hour drive up to Disneyland. Use the Disneyland calendar (Is It Packed?) To pick days of the week to go when it's smooth and there are way less crowds!
So many amazing hikes, national parks, well known outdoors sports/campsites that make it perfect for outdoor adventures
I would recommend the Oceanside Pier, it’s an amazing view and tons of activities to do by the beach. Just walking on the sidewalk, there is much to see, even open houses to view.
I am here once a year for Comic Con and love being down in San Diego for that weekend. The Gaslamp section is fun and exciting and trendy.