I agree that gramming volunteering into 2 months doesn't look good but spending some time while in NYC for the summer looks fine as long as you continue with something service-related, non-clinical on into the Fall and Winter in whatever place you end up.
If you are only there two months you don't want something with a large training requirement or where a longitudinal relationship with a buddy/client is desirable.
NYC is a big place. Can you narrow it down geographically? Are you new to NYC or "going home" for the summer?