Besides a fiddle, bow, shoulder rest and rosin, I have:
An electronic tuner
A little snake thingie with a sponge in it that you stuff in the sound hole to humidify the fiddle in winter.
A bunch of strings. Some are actually fiddle strings.
Sometimes there's sheet music in there, so in slow moments I can try to learn tunes.
Spare car and house keys. (If I'm locked out. I'm likely to have the fiddle with me.)
A Gary Larsen cartoon with a guy sitting in an orchestra with a violin, looking at his music stand and saying, "Gee...look at all the little black dots."
A nose flute.