Visiting the US from Australia - Car insurance/rego etc

I think your toast in these departments and won't be able to do very much. I found a few links worth reading anyways. I would think your only way to get a vehicle is to have a family member living in the US that would let you use their car (even if they had coverage) for under 25.
Good Luck.

http://www.hm-usa.com/carrent04.html

http://www.bnm.com/uage2.htm