Long Beach is a place with a strong military presence and history. I imagine that this affects police behavior and attitudes, and that the locations where a 'diligent' officer could use their power expand immensely in such an environment.
It's been 5 years or more since I've had any building security approach me in San Francisco. Know your rights, know when you are on public land, know to ask them if you are on their property. Beyond a little mental preparation, I'd be really surprised if you have any problems. I do both birding and photography across the city, so I often have both binoculars and a camera while in port areas, near schools, parks, etc.
As a matter of course, I do not shoot children in public without explicit engagement and permission from an adult. If I was going to photograph at a playground, I would engage parents and such beforehand, etc. It is a drag, of course, that such paranoia has become accepted, but in this case I go along with the dominant mood.
San Francisco is full of tourists. People have cameras everywhere. If you ever feel that someone is confronting you needlessly, go into 'innocent tourist' mindset and it will all be fine. If you have a Southern accent, even better! Play it up. When I want to shoot in the street downtown, I put on a touristy outfit, and I can do almost anything. Wear a sweatshirt that says 'San Francisco' on it and you have the closest thing to diplomatic immunity you will probably ever experience.
Depending on what kind of camera you are using, learn a bit about various neighborhoods. There are few truly dangerous areas, but there are places where I don't like being alone carrying a few hundred or thousands dollars worth of easily-transported materials.
Tons of 'youngsters' (20-30 year olds) think film is cool. Most conversations will be about your camera, film, etc., not about whether you should be arrested or not. Be respectful of parents' concerns, be curious and polite to others, and have a good time.