We found this article at Kristen's Guide the most inspiring in terms of ideas for what to include in your kit. First of all, you can find a relatively affordable working stethoscope and working otoscope. If you learn how to use the otoscope, you might even save yourself a trip to the doctor's office.
Then, consider printing out an eye chart and X-rays from the Internet.
The rest can be picked up at your drugstore: cotton balls, popsicle sticks, a medicine dispenser, ace bandages, gauze, and a pack of bandaids.
If you want to go even simpler, you can make the case shown above and throw in just a few supplies. We found the directions at Baby Center.
(Image: Counselor Grace/Baby Center)