Some airports are free to activate for an unlimited amount of time. Most of these airports are payware even though they are free to activate. But as you will see, ISD's airports are included (only Verona is free to activate but I left the others in the list) as well as a freeware aiport off AVSIM.com. The full list of all Airports supported is
here. At the moment these are the ones free to activate;
Aerosoft German Airports:
* EDDN - Default FS2004 Nurnberg (0 credits, free!)
* EDDN - Nurnberg (0 credits, free!)
* EDLN - Monchengladbach (0 credits, free!)
* EHBK - Maastricht-Aachen (0 credits, free!)
* EDSB