For me practicing and performing music are distinctly different activities, but should both be fun. Practicing music is intellectual, thoughtful, and controlled. It is how we input sound into our imagination. Performing music is (ideally) an intuitive, playful recall and controlled execution of what we hear for the purpose of communicating to an audience and the musicians we perform with. The fun in practicing comes in sensing the control we are gaining over the elements we are trying to master, moment by moment. The fun in performing is in the joy of reaching people. But these pages are dedicated to practice techniques.
To me, all music practice is about selecting a musical problem (e.g. how do I play this phrase with precision, relaxation, a good sound, and a strong time feel.) and solving it. The solution requires focused correct repetition to imprint it into your fingers and musical imagination, so it eventually will be accessible to your ears and memory. Therefore, good practice is also ear training, and slow, perfect repetition really helps it stick. This improves your musicianship at the same time.
The problem that you select should target a deficit in your playing. Remember that though the problem may begin as somewhat theoretical and abstract, it is still sound, and it should be a sound that you find attractive enough to study. There is no point practicing something that does not resonate with your imagination. The problem should be distilled into something small which, through focused repetition, you can watch improve in a short period of time. For me that is usually about 5-10 minutes, after which my concentration begins to wane. If I am sensing no improvement, I simplify the problem by working with less material and/or at a slower tempo. The satisfaction comes in sensing the improvement and feeling more in control.
Here is an example of the process that I apply to everything. To master a II-V-I voicing progression, perfect it in one key center before attempting the other eleven. First play it correctly. Next, as quickly as possible, bring it into the realm of time by repeating it with a metronome and adding a comping rhythm to it. Make sure it is still correct, and continue repetitions, gradually moving your eyes from the page to the keyboard, then away from the keyboard altogether (maybe closing your eyes.) Then continue correct repetitions, focus on relaxing your shoulders, then focusing on the sound, and then focusing on your breathing. These are a lot of repetitions, many more than just playing it correctly once and moving on to something else. Externally you are playing the same thing over and over; Internally, it should be changing for you. The idea is to gradually remove conscious supervision of your hands, so they seem to run on their own. In a short time, the progression should become strong and consistent, and your ears and fingers get to know it very well.
You can then increase the tempo or go to the next key, which will likely come more easily. When your concentration is flagging, take a break, or select a different problem you want to solve. In this manner you have been practicing more than just the II-V-I progression; you have been developing focus, time, and sound, which will apply to everything you do. This is how I practice everything.
Ever wondering what to practice? The following download is my idea of a well-balanced organization of practice time for a jazz pianist.