It’s top secret but the Airport Open Weekend is actually free admission this year (long story!). Damien’s right about there being less attractions such as funfairs etc. but on the plus side, there is far less chance of a 3-year-old getting lost at the Airport.
When the show starts there is a pretty constant stream of aircraft movements and you can get very close to the action. An added attraction close to my heart is the Vulcan taxi-run after the Typhoon has landed on the Sunday afternoon – 1700 local-ish.