The following “meditative practicing” technique comes in really handy when far away from your instrument.
All you have to do is choose a notated piece of music that you’ve got memorized and try and play it in your mind as precisely as possible, hearing it, and even feeling the touch of the piano (this also works if you play another instrument: just mentally recall the feeling you experience when playing your particular instrument). For pianists, Bach’s Inventions work well because they consist of two contrapuntal parts, which are already challenging enough to hear simultaneously with the mind’s ear. But you can choose virtually any piece of music. When a passage seems unclear, go back and “replay” it again, slower if necessary (just as you would when you practice on your instrument) until you’re able to hear each note, as well as each item of expression attached to each note, with utmost precision.
This technique certainly requires sharp concentration and thus works best in a calm environment. But if practiced correctly, its benefits are certainly to be felt as soon as you return to the piano (the following day for example – it’s always good to allow the mental exercise to fully sink in during the night…): overall, your knowledge of the piece will have been considerably strengthened; your memory won’t fail you and you’ll be able to concentrate on musicality right off the bat.