Cars? Hmm.. a convertible would work most probably. The weather isn't that bad in California -- and unless you're in the Sierras I'll lay money you'll never see anything more than a bit of rain or mist. If you're concerned, and with over 3 weeks in the car you might go for a somewhat larger car -- enough to put a small cooler with snacks and soft drinks inside, or stash that special case of wine in the trunk (boot) with your luggage so you can open a bottle in the evenings after the drive is over.

We've had lots of discussions on here about what would be the best car, and it all comes down to personal perference. I tend to go for a slightly larger car -- more room so people can stretch out a bit more on a longish day's drive, and so your gear isn't crammed into every free inch of speace. Some folks swear by minivans (plenty of room!) or SUVs (what I use) or smaller cars with much better mileage. It's all up to your taste in what you'd like or feel comfortable in -- with your gear and your family for 3 weeks on the road. You shouldn't need a RV, which would be a specialty rental.

Rental cars & Insurance. I've used a bunch of the large national chains and they all seem to be pretty good -- lots of locations (Just about anywhere there's an airport), and some even offer the option of picking the car you like on the spot from a number of those available. Check the price and other terms and see what options you like.

All of the major car rental companies have web sites which should give you an indication of types of vehicles they offer and costs to rent them. You should be able to reserve a specific type of vehicle for a day and location to pick it up, but I've run into situations where the rental agency was very busy and they did not have the specific type of vehicle available I wanted. I'd do a quick check on prices and options -- and there have been a couple of other companies folks from Europe have recommended in the US, that specialize in rentals to non-US residents, which I'm not familar with at all.

But for insurance, you might want to give your local auto insurance company or agent a call. You are required to have a certain minimum level of insurance to drive in California, and this is one thing the rental agencies check before releasing a car for you do drive. You might find you home company can provide coverage in the US through either a partnership agreement with a US firm, of direct coverage. For example, my auto insurance company claims they provide some level of coverage no matter what car I'm in -- including rental cars and foreign driving (different levels of course, but they offer some options to give me expanded coverage for driving in Europe, for example). You might find that you can get much cheaper insurance through your local agent, than the expensive insurance offered through a rental agency. You will need to have something to show proof of insurance to rent the car, and also if you get stopped for a traffic violation in California. (Piece of paper giving who your insurance company is, the policy number, the dates your coverage is good for, and some contact info, like a telephone number).

Similarly, if you belong to the Australian Automobile Association, they are the sister of the American Automotive Association, which offers insurance, roadside assistance, and other services including travel arrangements, discounts and maps through local clubs in the US (such as the Automobile Club of Southern Californi) . You should check them out to see if they have some options to make like easier for you. A quick look at their web site shows they offer "International Driving Permits" which may getting a rental car (and showing you have a valid drivers permit), and they may be able to offer some type of international drivers insurance or point you at some company which offers this. Some of the clubs have reciprocal agreements for roadside assistance, use of their offices for maps and the like, but without checking I don't know what arrangemetns the AAA might have with the AAA in the US -- If you're a member, check out what they might have to offer.