|Minimal sketch kit|
Sketches can be done in pretty much any medium that you can think of. (And possibly some that you can't.)
It's largely down to what's available. But if you are reasonably well organised, you can make a point of carrying a few items around with you, and so making your favoured implements available when required...
I usually carry a small sketchbook and a few stick-like implements (pens and pencils) around with me, but planned sketching expeditions can be more expansively catered for by packing sketching stuff into a nice, big bag (a satchel is a good choice).
Minimal sketching kitillustrated above (in Uniball gel pens on black paper)
This consists of a small (usually hardback) book and a pen.
Sometimes I take a pencil instead; sometimes I expand it slightly with another pen or a small pouch/case/tin of other stuff. I like to vary the other stuff, but there are a few standby collections, usually involving colour.