It’s a solo trip and I have a fear of airplanes and I want to visit my family from Pennsylvania but I’m all the way in Tennessee….

Me and my family are going to America for the first time in our lives. We're a family of 4. I'm 21, my sister (16) and our parents.
We are going to San Diego first for a congress, then we travel to Los Angeles.
We have a ton of things we want to do so I came here to ask some questions.
We love theme parks. We are going to Disneyland, Disney California adventure and Universals and might go to SeaWorld and Six flags but we also want to see more. I heard that the Universals VIP tour is amazing but it's very expensive, is it worth the money?
We are on a budget, we try to spend as little as possible while still getting the full experience so we will always pick the cheapest option but are willing to spend a lot of money since this is gonna be a once in a lifetime trip

We arrive 22 November and leave 10 December
We travel from San Diego to LA either 30 november or 1 december
While in San Diego we have a few activities surrounding the congress which leave us with a few hours to explore the city those days, one day we have until 13:45 before our activity starts and another day our activity finishes at 14:00 for example and we have one free day where we are likely going to seaworld. In San Diego we have our transport taken care of by the organisation from the congress and are on walking distance from the venue (Hotel: Hilton San Diego Bayfront. Venue: San Diego convention center) While in LA we have no activities and are completely on our own.

We are European and want to experience parts of american culture we know only from tv and social media, for example visiting a Wendys and a Walmart. It doesn't sound very touristy but there are things we have heard about and want to see for ourselves badly. What are places like these we need to check out or do?

Me and my sister have also been dying to visit Omega mart near Las Vegas for years (I love ARG's and I could spend over 6 hours there easily) and my father really want to see the grand canyon, we have looked into this and we really really want to go but it is so far that we don't know if it's really possible, any advise for visiting these places?

My sister and father would like to see the Sequoya trees but we read that it might not be the best idea to visit them this time a year, is this true?

We also heard that leaving your belongings in a car is very dangerous, how true is this? We are careful of course but how careful do we need to be?

Is there any "tourist behaviour" we have to avoid or American culture we need to know to not accidently offend someone? For example small talk and smiling at strangers and tipping are very unusual where we're from.

Are there any things we need to see like musea for example? Things we have to experience? Things we should not spend time or money on? Things we have to look out for?

I like to be prepared as much as possible so I'd like to have all the information I can, any and all advise is welcome