Take a catheter bag, slit along the top; fill 3/4 with water, some dark yellow food coloring; and 2-3 small goldfish (good contrast with the colored water)
Take a patient that is either coherent with a sense of humor, or unconscious. Run the tubing under the covers about the waist area.
First one; took a patient from ECF to X-ray at local hospital; on the way going through the ED, we stopped a couple of nurses; asking if they thought our patient should be checked for a UTI. And Doctors, and other medics, and x-ray techs.
Did the same when we returned the patient to the ECF. Told the nurse that the hospital cathed the patient, and we thought he had a bad UTI.
Last prank on me on the job at ambo company had me pull the prankster through the pass-through window from the back to the front of the Cadillac ambulance. I'm not in favor of anything physical unless it's going to be immediately funny to the subject too.