Basic menu with single player, multiplayer, controls and exit buttons
For single player, provide a car selection screen (cars with some sort of garage background, car stats on images), pan or rotate camera in 3D to display cars.
For multiplayer instantiate another camera and change the settings for split screen layout, then have the same car selection screen as for single player in each normalized viewport rectangle.
Controls can display volume (sound fx and music separate), graphics detail level with adjustable sliders, along with an image/text listing the controls for the game
And and exit button which is obvious. All of the above controls should also have back/exit buttons for each section.
One the player(s) has selected a car, show a start game button. When clicked, load the game level scene.
For single player, load GUI, do a 3,2,1 countdown and start the timer and allow the player to move.
For multiplayer do the same but instantiate another car with camera at the starting position, and change the normalized viewport rectangles to split screen settings