There are many aspects of practice which you can complete without your instrument. For this technique you will pretend that you are one of those mime artists that you see at the circus. You pretend that you are holding your instrument and continue to practise like that. You can practise things like fingering, breathing, embouchure, bowing and many others all without your instrument.
Just because you are miming however does not mean that you can be sloppy with your playing, Just as when you have your real instrument you need to make sure that you do everything as accurately as you can.
If you want to take this one stage further there are many other aspects of music you can practise without and instrument. You could for instance try to memorise one of your pieces or you could do some theory in your head (what are those notes that make up a D minor chord?).
The possibilities for practice without an instrument are endless!