1. Play your P-120 through a sound system other than the one built into the keyboard (computer not necessarily required)
2. Conrol sound samples running on your computer. I use my Yamaha PF500 together with GarageBand on a Mac. Even the puny piano sound samples that come with this $49 software beat the built-in sounds in many aspects. Connect it to professional samples, such as one of the magnificent Bösendorfer 290's, and you won't believe your ears (well, compared to the built-in sounds of a P-120, that is, but if you have a really good sound system, then it even compares to the real thing).
3. Composing. Advanced notation software (e.g. Sibelius) writes down exactly what you play in real time - some editing required - download their trial version.
4. Practicing. Some software lets you play from score displayed on a computer and follows you, noting mistakes and scrolling ahead when necessary; no page-turning required
5. Play a MIDI file through your keyboard. Download thousands of MIDI files and have them reproduced with your favorite settings. One can find all kinds of pieces that others have recorded, from simple ones to entire sonatas and concertos. Check out Sibelius' Scorch system: you load sheet music into Sibelius and have it played. Such music is exactly reproduced, so it lacks color and all the emotional aspects, but it can help a lot to figure out a piece; e.g. you can play only certain voices or slow it down and so on. Mind you, one does not need a keyboard for that.
6. Recording. Record either the original sound that the keyboard produces (make mp3's), or just store the MIDI information. It helps to hear oneself play. Using a digital piano for that is easier than dragging a microphone around.
7. Make CDs and sell them for big bucks. Send your renditions to family & friends or to your teacher asking why that phrase sounds so crappy and how one should really play it. I use GarageBand on a Mac for recording. If I make a mistake, heehee, I move the note around a bit to change its pitch, duration or loudness, whatever...
One can also assemble whole symphonies by using the keyboard to control sound samples of pretty much any instrument. A lot of (most?) film music and loads of pop music is done that way.
I would suggest using Google to get more info. There is tons out there. Have fun!