I found collaborating with congenial doctors about problems that physicists could help solve was very satisfying. I also like educating anybody who would listen!
Most medical physicists work in the physics of radiation oncology making sure that the desired dose is given to the cancer and the dose to normal tissues are minimized.
Medical physicists work in cooperation with doctors. A few medical physicists devote their time to research and teaching. A few get involved with administrative duties.
The growth of technology is such that it is not possible today for a nuclear physicist to switch into medical physics without training. The field is now much more technical. More training is needed to do the job.