What should you draw on a cast?

Casts are a great surface to draw on.
If this cast belongs to someone else, draw something fun and happy, that will remind your friend of you. Small animals, flowers, or caricatures are great ideas. Keep your work small enough to allow for others to draw, sign, or write messages. Remember that the cast will be cut off at some time, so extensive work will "disappear" once the cast is gone, so get lots of good photos of the work.
If the cast is your own, then you get to work on it pretty much as you like. Remember a couple of very important factors.
1) Who is your work for? if it is just for your own fun and enjoyment, you can work in whatever manner suits you. But if you want others to see it, you should work so your drawing can be seen (and appreciated), and take into consideration where other will be looking at the art. If this is a leg cast, be sure the work will be right-side-up, when you are standing...
2) What kind of cast are you working on? Plaster is easier to work on than fiberglass. It is smoother and takes colour better. Coloured markers work well on plaster and acrylic paints are great on plaster too. Fiberglass needs paint (acrylic works best) to have your work seen, because of the texture of the fiberglass markers are not as good.
3) How long are you going to be in THIS cast? Often casts are replaced during the healing process. A lot of hard work can end up on the floor of the doctor's office if you don't plan ahead.
4) Take LOTS of photos and if you want to keep the cast, check with your doctor, usually they are good about it if you let them know in advance.