Any medical school will have a Standardized Patient program, and you can probably sign up with it. What is a Standardized Patient? Well, it's basically a person from the community (ie not another medical student) who has been specially trained to act like a person who's come in for help. You get a dossier about 'yourself', where this patient lives, works, eats, who they live with, how they got the injury, any symptoms they're experiencing, etc. This is to help train medical students on how to interact with patients. Sometimes they may be practicing how to take a physical exam on someone who has a cracked rib, or telling you that you have an inoperable brain tumor. I think this is a really valuable program, and apparently so do some other people since it's now part of the USMLE (the US Medical Licensing Examination, which all new doctors have to pass.) They must complete approximately 12 patient encounters and are evaluated on each one.
Part of the value in this exercise is that after the exercise is over, you can give the students really valuable feedback about how their approach worked, and what they might do differently. Practicing on you means that they'll have a lot better idea how to actually tell someone they have an inoperable brain tumor if the time ever comes.
The big medical school in my town pays $10 an hour to train and $12 an hour to work with the students. You'll need to play the same part over and over again, so it can get a little tiresome, or on the flip side, you could get really into playing a sprained wrist! I think it's also a valuable service to the doctors of the near future. Any adult can do it - in fact, they need a great diversity of people since youwill be matched with pre-written cases that take into account your gender, ethnicity, weight, own previous health history (ie you can't play an appendicitis case if you have an appendectomy scar) and other factors. Whether or not you know anything about medicine is also a moot point, since the people who come into a hospital usually don't know either.