One of the best "non-essential" systems we installed in our boat were our cockpit speakers. I don't know what it is, but being able to sail along effortlessly on a beautiful day while listening to some well appointed tunes (think Jack Johnson, Zac Brown or if you're feeling particularly pensive, some nice downtempo) makes that sail even more enjoyable. Furthermore, relaxing in some beautiful anchorage with a sundowner in hand just isn't the same if you don't have some nice background music to accompany it. That's how we feel about it, anyway. So cockpit speakers have always been, and will always be, a must for us in our boats.
We also installed a Fusion marine stereo receiver with built-in iPod Dock and absolutely love it. Our iPods insert directly in the unit, keeping them safe, secure and free from any cumbersome chords, and unlimited music free of interruption is ours. We took our music control a step further by installing the Fusion marine wired remote control which is mounted our steering pedestal so we don't have to run up and down the companionway to change a song or switch out iPods. It comes in handy if that particular Brittany Spears song starts playing when we have company; we can quickly change it and pretend like we downloaded it only for Isla's dance parties (cough, cough).
One good thing about music, when it hits - you feel no pain. - Bob Marley