What to use for sketching on a canvas?

Charcoal would work well, but if you plan on not painting lots of layers anything heavy would show through. You could always use darker pencils (softer lead) like a 4B(normal writing pencils are typically HB). Any art store would have them and you could ask the staff about what they recommend.

Charcoal will bleed through your acrylics, but it helps if you start with a wash just to give yourself enough information before you lay down the heavy coats. Once you start to add paint to your surface in thicker layers, the charcoal basically disappears. I used to try to avoid that by using soft led pencils, and that is OK except for the fact that they are tougher to use on a textured surface. It also turned out not to be as fun! I will keep an eye on this question because I'm sure someone will share some interesting concept that I will try to use myself!

Canvases aren't meant for drawing, most of what you put on there is just going to come off. You could use oil pastels, they might stick best, but there's no guarantee. Next time just buy some cold press paper.

charcoal :)