Instead of renting, you get a short-term tourist lease
. Comprehensive insurance with no deductible, you get to order a brand new car with whatever options (diesel, child seats, racks, whatever) you want, unlimited mileage, minimum age 18 instead of 23 for renting, no taxes, roadside assistance, don't have to drop it off in the same place you picked it up, and you save from 20% to 50%. Minimum lease is 17 days.
Why it works: Tax. New cars get a large tax, but one that's just off lease does not. When you're done they can sell it at new price and the buyer gets a big discount for an almost-new car.
The linked article has more details, links to lease companies, and comments from other people with other ideas.