So after a scattered early morning text session about whether there were enough rental cars, how to get there, etc. we were off. I was nervous about driving on the left, but it was easier than I expected. It just required a lot of concentration. Like the equivalent of driving a bumper car or a moving truck--lots in common, but you have to focus a little more.
We ended up visiting 4 wineries on tandem bikes. It was as fun as you're picturing right now. As you can see from this map, everything is very close: