What a great question. I also have a little (ok not so little) bag for such occasions and, being of the girly pursuasion, a handbag with stuff in it that would make some people's hair curl!
In my little bag or just wrapped up in a scarf (which doubles as an altar cloth):
Candle and means to light it (matches or lighter)
Herb of preference as incense
Two small brass dishes for offering and water
4 x specific small stones for the directions.
Either a tiny rattle made from a mint tin with seeds in or two old croital bells.
So not much really, it all wraps up into a 100cm square but enough to set up anywhere and do the basics. It can be strapped to the inside of a drum and taken along with that too.