Take the car rentals liability insurance in that case.
Alternatives are to use zipcar which provides liability inclusive for it's hourly/daily rentals
http://en.wikipedia.org/wiki/Zipcar
or to find a discount code that offers liability. For example some companies include liability coverage (ask your company if they have a corporate rate with any car rental company) or if you are a USAA member then you can get liability included at no extra cost.