In addition to improving airports by editing the layouts, many airports can be improved by
adding objects with the UFO. In difficulty level this is to editing the layouts what creating liveries are to making an aircraft.
Apart from these more direct ways to contribute, do not forget the indirect ways, like being around the forum or IRC trying to find users with problems you can help or improving the help for beginners. I am pleased to see that you already have a wiki account (and a few edits).