1 player. Choose a simple familiar tune. Yankee Doodle, for instance. Figure out the notes by ear. Play it through again and again. Each time through, change something about it. Make it a small change. What you are doing is not so much making variations of the tune (which you are, for a while), but rather you are very gradually transforming the tune into something else, into a new tune. Make the tune very simple and familiar and the changes small so that you can remember what you did the last time through and change it only slightly each time. But keep it up; after a while the tune should be utterly transformed.
Ideas: Dynamics. Articulation. A note here and there. Rhythms (note values). Register. Graces notes. Turns. Glissandos. Fill in leaps. Add sharps and flats. Style. Add rests. Make major minor and vice-versa. Add extended techniques. See how long you can keep this up, either times through or time (duration). Make a note of it. Try again every day for a week. Send me an email with your best time.
Repeat in a new key.
Repeat with a new tune.
Add a partner now and then.
Keep doing this for many years.