I play exactly what I feel like to play. I ponder a bit over what attracts me the most today, and then I work with that. If I'm in an ambitious mode, which I often am - yes, really! - I start by planning my work with this particular piece. Should I concentrate on certain tricky spots/sections, or should I try a run-through? Sometimes I experiment wildly with tempos, omitting certain notes and other stuff. Sometimes I try to stick exactly to the written notes.
Sometimes I feel like doing technical exercises. Sometimes I just read the music and test the fingering out.
You might think this is extremely wishy-washy and unefficient. The traditional view on practicing is "no pain, no gain", right? If you are SERIOUS, you should be SYSTEMATIC and word HARD etcetera. But one thing I learned, many many years ago, is that this engine cannot run without its fuel, and the fuel is MOTIVATION. Anything to keep your motivation up, that is my slogan. So, I aim to have fun at the piano. And I have proven that my theory is right, because dramatic things have happened since I started with this approach. Yesterday I had a lesson and at the end my teacher was like "I cannot believe how much you have learned! And I have never seen anyone practice with such devotion and determination as you do!"
I did not write this as a gesture of stinking self-praise, but again: this is the result of just amusing myself at the piano. Back in those days I worked after a schedule, I made very little progress and finally I lost my interest in piano studies, and I was convinced that I did have any talent.
Some days I just don't feel like practicing at all, some days I work for hours. I go with the flow!