All the same ones that you need for M.D. schools. These are not required, but are good to have (in no order):
Research
Volunteering
Leadership Experience (organizations, clubs, societies, student government, etc.)
Shadowing*
Miscellaneous Extra-Curriculars (Mission trips, sports, hobbies)
*Any shadowing will do, but shadowing a D.O. is preferred because some D.O. schools require a D.O. letter of recommendation.