Mick,
I just got back into town. I would echo the suggestion to use airbnb.com and vrbo.com These tend to be MUCH cheaper than hotels and have full amenities of apartment/condominiums. The upside is that the girls can buy groceries and cook a few meals to save a bunch of money, instead of going out for every meal. Between Los Angeles and Orange County, they should look at Santa Monica, Marina Del Rey, Manhattan Beach, Hermosa Beach, Redondo Beach, Newport Beach and Huntington Beach -- all great beach communities and generally safe neighborhoods. I am not sure how California beaches will stack up to Australia but those cities all have great beaches and commercial centers (shopping, clubs/bars, restaurants, etc.). Most tourists like to visit Hollywood, but truth be told most of it is seedy. There is good people watching in Beverly Hills and Venice Beach in LA.
Amusement parks are also plentiful: Disneyland (in Anaheim/Orange County); Magic Mountain (good thrill rides); Universal Studios (has a fun tour theme park on the entertainment industry). Knott's Berry Farm is a yawn as is Lego Land (better for young children). San Diego has Sea World, as well.
For transportation, their age might be a factor. Most car rental agencies will not rent to people under 25 years of age. They will need some form of credentials to drive, too. If they are not renting cars, they might still be able to rent scooters but that will not get them onto the freeways. Another VERY popular transportation option is UBER. You can get an app on your iPhone or other smartphone. The cars are all safe and the fares include tip. This is the best way to get around San Francisco in the city. If they are flying into San Francisco, they should take the BART (subway) direct from the airport to the city; it will save them a good chunk of change versus a taxi.
In Vegas, there are tons of hotels. I would avoid staying in Downtown Vegas (it's a little seedy), and stay on the Las Vegas Strip. There is a free monorail that travels the majority of the length of the Strip. If they are under 21, Vegas will not be as fun, since the drinking age is 21 here; gambling is allowed for 18 years up.