Play MIDI music files other than the ones pre-recorded into your keyboard, by inserting a MIDI music disk. You may save your tunes onto a disk too. Many keyboards have neat special effects functions. Play with your music by adding echoes, looping it, reversing it, and other fun things. Special effects functions vary from one keyboard to another.
Drum pads are also common on many electronic keyboards.
Press one of these and a percussion sound is made. Many keyboards also have split keyboard functionality where keys on the left hand side become cords in certain modes. This is usually not exactly a stereo output but a BiPhonic output. There's always room for expansion. MIDI input and output ports are also very common now.