Now this is a great idea! I just heard about Napa Valley Designated Drivers. They will provide you with a designated driver for your own or rental car so that your entire group can do a few hours of wine tasting safely. It looks like they charge about $50 an hour which is less expensive than hiring a car and driver, taxi, and much less expensive than a DUI.
I’ve never used this service nor do I know anyone who has, but they have good feedback on Facebook and Yelp.