Use your experiences to set you apart. explain how great the head TA experience was. i did something like that and got to say how much better i learned the material when i had to teach it well enough for others to pass the class, rather than just learn it well enough for myself to pass the class...
and the volunteering is good. when i interviewed, my only real experiences medically were 2 summers of near-full time research, a 1-week job shadow, and about 7 months of volunteering @ 4 hours/week. so if you did the volunteering for 6ish months at 4+ hours/week, i'd think you're looking golden.