Chain tool

[silva]

And hold it in a plastic bag big enough to fold it over a couple times then no rusty surprise the day you need it. It also prevents the oil from gradually evaporating/drying up.

 

[Nigelnightmare]

And hold it in a plastic bag big enough to fold it over a couple times then no rusty surprise the day you need it. It also prevents the oil from gradually evaporating/drying up.

Mine lives in a "Tesco's" small resealable bag with some spare chain & quick links + chain hook made from an old spoke.
In fact all the tools are in those resealable bags, it stops them getting wet, prevents any muck getting on anything else in the pannier bag AND they're all in one place.