Your best bet to develop a spike starts by looking at areas where you're already highly ranked and willing to put a lot of time. Spikes do not have to be strictly in the area of your future profession (surgeon).
Take a look at what you already rank well for. Is it math? Science? (Intel Talent Search, Olympiads?) Research? This is the most fruitful source.
Your spike does NOT have to relate strongly to your future career. If you want to be a scientist later, but you're really good at writing short stories and getting prizes for them, there is nothing wrong with getting into college based on your short stories and then aiming for your career afterwards. I went to Harvard, my career after college was Economics research, and I got into Harvard based on a spike in Chemistry and Math. One of my roommates got a job as a top consultant after college, and he got in because he published amazing cartoons. (However, if your entire resume's story is to be a surgeon, it probably helps lightly to moderately to have a spike relate to it.)
Common spikes I see for people who ended up getting an MD include:
- Doing really well on research (Intel Talent Search)
- Math and Science Olympiads.
These probably won't get you far:
- Volutneering a lot at hospitals.
- Shadowing a surgeon around.