I am creating a music-based app for one of the courses I teach at a high school, so it's not for sale in the app store, it's only for my students to use. I decided to make it keyboard only partially because I didn't want to have to create a whole bunch of buttons and then link them to code. What's a good way to indicate on the screen what certain keys do? For example, the escape key will exit the user from the settings menu scene, it is also used to bring up a list of options in-game. Is there a slick way to explain what certain keys do without having some sort of legend on screen in the various scenes? Screens = scenes (I'm using SpriteKit for OS X). Remember, using the mouse is not at all an option.
You can add an "implied tutorial" like @Anko suggests, that's a good idea for games where the player will find many type of difficulties within the campaign.
For a simple game, where you always have the same mechanics and those are really simple (for example, a fight game, or an infinite runner) you may use a "help" label (for example, "Press 'H' for Help", always shown on a discrete way either on the game scene (top right corner, for example) or in the "start" scene. When the user presses this "H", it will show an image with all the other possible keys.
I think the second option fits you better (and it is really easier to develop).