Mine would have to be giving her a physical exam and then (fake of course!) anesthesia.
It starts out that she walks in, dressed in athletic wear, asking for a physical exam. I am dressed as the doctor. I tell her that it's almost time to close the clinic, and there is no chaperone available to stay with us while I do her physical. I ask if that is ok. She says yes.
Then, I start with the physical exam. To make a long story short, she either plays "nice" or "naughty". If she is playing "nice", she is cooperative. friendly, and even a bit flirty with me. When I get to the part where I have her strip to her waist and start to examine her heart, our eyes meet and she is getting pretty excited. She grabs me and kisses me. We have a long kiss while I listen to her heart speed up. Then I put her feet in the stirrups and we make love right there on the exam table.
If she plays "naughty", she is stuck up, a bit rude, and noncompliant or hesitant to comply with what I ask her to do (taking off clothing, doing what I ask). So I do the lung and heart exam, and then I ask her to put on the "oxygen mask" which is really an anesthesia mask. I tell her I need to check her VO2 max for sports, and she reluctantly agrees. She begins taking in deep breaths as I (pretend) to first turn on the oxygen, and then the laughing gas. In a few breaths, she pretends to become very giggly and silly. Then I (pretend) to turn on the anesthetic gas. She takes a couple more deep breaths while I listen. Then her eyes get a blank stare, then she blinks slowly and her eyes start rolling around in her head. She pretends to become very sleepy and I lay her back on the table, listening to her heart and lungs as she breathes the (pretend) anesthetic gas. In a few breaths she pretends to fall asleep, and I strip her the rest of the way naked, put her feet in the stirrups and make love to her right there on the table.
So my 2 favorites would be the stethoscope and the anesthesia gas (but only pretend - NOT FOR REAL!)