One of the handiest bits of kit for quick messages and notes is a bit of light colored laminate of the type used for counter-tops. The matte surface accepts pencil marks and erases with a little spit and a quick wipe. A moist cloth for the civilized.

I've sometimes glued a 3" square piece to my tape measure for quick notes. A couple 3" by 5" pieces make a handy permanent note pad. When I was doing orienteering I would figure it all out on this. After the fact I would transcribe the pertinent information to a regular log.

Small pieces of laminate are often handed out as samples but any cabinet or counter shop that handles the material can hook you up with scraps that work well.

There are also sheets of textured Mylar laminate that are available form shop specializing in drafting materials. This is kind of like very tough pieces of paper. You can make permanent marks on this material using special solvent inks and markers. This can be handy for frequently used formulas, grids, important instructions. pencil, grease pencil, and some types of ink, can be easily erased but the marks made in solvent ink melt into the Mylar and have to be scraped out of the surface of plastic to be removed.