CHARCOAL MARKERS & SPRAY PAINT

[TygerxEyes]

I am not sure that this is a possibility... however, the ability to mark cave walls would be nice (wink-nudge-tease). It would be a lot of work to have people be able to write things... but you could have several different colors and symbols - so when a character uses the can they can choose between one of a few colors (e.g., traffic lane yellow and white plus blue and red seem common enough and then choosing between a few symbols could be done with a little ease (e.g., arrows :  / daggers : † ‡ / skull/crossbones : ¤) would do the trick.

[CalNieDaGtarGuy]

It sounds cool but its practicality seems strange for people who know their maps. You could use it for role-playing purposes, but I dunno.

[Wasteland Watcher]

 Though convenient, not necessary. 

 Since the beginning I have always used sticks to put arrows on the ground in caves or wherever so I never lose my way. 

 Charcoal on the cave wall would probably not be very visible, and this will sound funny but the thing I have against spray paint is that it's basically vandalism