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