There are almost always more patients than personnel and supplies. Your talents would be well put to use in this type of setting. But, even if you don't have a medical background, you could still lend a hand. Where there are patients, there will be records that need to be filed, phones to be answered, and an assortment of other duties just like those in the traditional health clinics and doctors' offices. Even other backgrounds can lend themselves to golden opportunities for service.
Retired teachers can become tutors, or volunteers in schools.
A former law enforcement officer could offer home security seminars, either individually or as a service to senior groups. If your earlier life was spent in construction, so many opportunities await you in retirement. Help out in building a Habitat for Humanity house, and bring home ownership to those who otherwise would never have such an opportunity. Whatever your background, skills and experiences, you have talents to offer. But you aren't limited to volunteer activities.