I've been searching for a while on ideas to be a bit more creating in first aid. We have, of course, done drills with Halloween supplies including fake blood. In the Wilderness First Aid classes, the instructors have stage makeup to simulate cuts, scrapes, and bruising, and that was what I started out searching for. Enter the joys of internet searching....
It took only a few minutes to come up with several sites that have good information on low-budget mock injuries and I learned a new word: moulage.
Brian Potter provides a few tips, the most useful (to me) is the idea for fake blood. The problem with most fake blood is that it leaves stains. That's probably possible for his suggestion, too, since it involves food coloring, but the base liquid is dish soap. Yep, dish soap. It has the right consistency and it is intrinsically washable.
Clark Green has a nice article with a couple of additional links on ideas and props you can build. Additionally, there a step-by-step instructions on simulating shock and heat exhaustion/stroke with just a bit of makeup, well worth reading through.
Last, and I love this one by "First Aid Dude" because there are detailed instructions for making reusable prosthetics. They may not be quite so realistic as something built directly on the "victim's" skin, but the ability to remove them and put them on someone quickly for a drill is worth while to me. Time for a quick trip to the local hardware store to pick up some flexible caulk.