Super Mario Galaxy is a cornucopia of fun, and in addition to the plethora of mechanics and levels that work better than they have any right to, it displays an understanding of how paying attention to the little things can make a game even more enjoyable than it already is.
For instance: In the level-selection menus, the galaxies that you can travel to are displayed on the screen. When you point at a galaxy, it lights up and a sound plays; nothing unusual about that. But as you point at different galaxies, you may eventually notice that the sound effect changes over time, staying in tune with the background music. Galaxies that you can’t yet travel to play a slightly different tone in a lower octave, which gives you two voices to play with in addition to the background music (which is just a loop of some ambient synth pads). I actually spent 15 minutes “jamming” with a menu screen when I discovered this, because I am apparently so easily amused that you don’t even need to give me a game to make me happy — a good UI is good enough for me.
This kind of pervasive musicality in a game makes me think of Classic Arcade Sounds, a collection of field recordings of 80s arcades. Most of these games used synthesized sound rather than samples, of course, and many of them seem to have sound effects that are nicely in tune with each other. Sometimes it almost seems as if the player isn’t even playing a game so much as a happily unwieldy instrument. Fun!