Tetris (Nintendo, NES)
Tetris is a famous puzzle game created by Alexey Pajitnov. This is the official licensed NES version, as opposed to Tengen's rendition (released six months earlier).
Having more detail is always a good thing.
Specifically: What needs to be modified to make these appear?
An airplane and helicopter can be found amongst the ending sprites. These aren't used in any of the endings of the game.
They do have valid sprite indices, and can be made to appear via hacking. The helicopter skids are cut off in-game due to programmer error and are thus unused, but the main and tail rotors are nicely animated.
The first track in the NSF is unused, and was presumably intended for the title screen (which is normally silent). It can be heard by setting RAM address $6F5 to 01 at any time. Alternately, use Game Genie code PESAZILA to replace music type 1 with this track.
Setting RAM address $BE to 02 (or using Game Genie code VUXEILXX) activates an unfinished two-player competitive mode, a rather surprising omission that was present in Tengen's rendition. The basic rules are in place and garbage attacks are fully implemented, but there are numerous issues:
Leftover Demo Recording Code
The game has an unused subroutine that records the player's input for use in the gameplay demo. This subroutine is activated when $00D0 in RAM is set to $FF. However, since the game tries to write the captured inputs directly to ROM ($DD00-$DEFF), it doesn't work correctly on the actual NES hardware or unmodified emulators.
Cheats in PAL Version
The European version of Tetris has cheats not present in the US version of the game. To activate the cheats, hold the Select button while a piece is landing. In A-Type, you get 10000 points. In B-Type, you get "Success" and instantly finish the level.