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.
(Image: Counselor Grace/Baby Center)

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We have all of the real stuff - but my kids get to play with the pretend. The trouble with a real stethoscope is that they think it's fun to bang it on things and/or yell in it and it about blows out their ear drums.
Still - I want to make this bag; the pretend stuff is in a hard plastic box and it takes some puzzling to get it all back in and snapped shut.
After our plastic one was recalled for the usual reason (Made in China, contaminated materials) I did this as well.
We got a small working stethoscope ($8) and other goodies from For Small Hands as well as some reference books. The otoscope is a great idea. I also made some play bandages that close with velcro.
Our doctor kit is made up of real things but children shouldn't play with a real stethoscope without adult supervision as they can seriously damage their hearing. We have one that we get out for science experiments or if we are playing doctors but only if the patient is mommy or daddy.
don't forget that little rubber reflex mallet!
Ohhh I love this. We have a table top light box (one of our favorite toys) that would be great to use with the x-ray idea.
We have a home-made doctor's kit. Instead of the bag, we keep it in a cardboard suitcase (like these: http://www.landofnod.com/kids-decorative-accents/kids-room-decor/pack-your-bags/f641), with a red cross glued on top. Much easier to store on a shelf than a bag!