Playing hymn tune in the left hand on a solo reed stop, alto and soprano plays the right hand, and the pedals take the bass;
Transposing a part of the piece half-step or whole-step up or down;
Singing one part while playing the others in a piece;
Improvising in a pentatonic mode (sharp keys only, for example);
Changing the stops by hand while playing;
Taking one measure of a challenging pedal part and moving up and down the pedalboard in this pattern like in an exercise;
Playing C major scale with the left hand and E major scale with the right hand simultaneously over 4 octaves up and down;
Playing with your eyes closed;
Try one or more of the above techniques today. It's less risky than you think it is. Afterwards you might even wonder why haven't you done this before. And of course, you will get other ideas along the way.
Don't forget to share your experience here in the comments.
Transposing Sequence in D Major: ii42-vii7-I