At the main menu, hold [Right Alt] + [Right Ctrl] + [Right Shift] and click on "Single Mission". Then, extra missions from older games in the "Jane's" series will be available. Additionally, the "Atomic Moth" and "Quetzalcoatl" aircraft can be selected.
More editor objects
At the main menu, hold [Right Alt] + [Right Ctrl] + [Right Shift] and click on "Create Pro Mission". Continue to hold those keys and click on "Objects" and "All" to access an extra page of objects.
Fly a helicopter
Enter the Create Pro Mission option and disable "Fly all aircraft". Then, return to the main menu, hold [Right Alt] + [Right Ctrl] + [Right Shift] and click on "Quick Mission Creator".
Extra campaign missions
Enter the Create Pro Mission option. Then while loading a mission, hold [Right Alt] + [Right Ctrl] + [Right Shift].