Something rediculous like this to turn it into a single seat roadster. Yes, I know this is a Jag D-type. And it was the best pic I could find at work.
It would be pretty interesting to see someone do something like that. Make a removable plate over the passenger seat to allow you to bolt in a seat, and some kind of rear plate to cover up and seal off the whole area b/w the back of the front seat to the trunk. With a roll bar/rear half cage and a streamline element, it could be pretty cool.