A good doctor kit will keep most toddlers busy for quite a while, but when we searched around a bit, there was nothing really great out there. Instead of buying a plastic bin of more plastic pieces- consider making your own doctor kit with authentic instruments. 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.