Abolsutely recommend Nurburgring. I flew Ryan Air last year to Hahn but then had to drive down to Frankfurt Main to hire a decent car (make sure you dont mention you're going to the Ring). You can get a Porsche for the weekend from Avis (check out avis.de\funcars) or Sixt have some nice cars...
SQ fly SIN-JFK via FRA so if you needed European connections you could always do something like SYD-SIN-FRA-LHR-FRA etc and then FRA-JFK. Then across to LAX-SFO and back to Sydney.
Cheers
Here's one to watch out for.
I rent all over the world all year long. Never had anything like this.
Last month in Sydney I had rented using my Hertz No 1 and the evening before returning the car I noticed one of the tyres was a little flat. I changed the tyre - turned out there was a nail...