Contact a state park near you and see what options are available if you are interested. Most large cities offer a wide range of volunteer opportunities. This site for the city of San Diego shows the different programs in San Diego. Any city will offer a similar set of programs. Look in the phone book and call around to see what might be available where you live.
Many city governments are large and fairly disorganized.
When you think about it, reading is one of the most important skills an adult can have. Many adults, however, have never learned how to read. Literacy volunteers act as tutors who help illiterate children and adults learn this important skill. Literacy Volunteers of Massachusetts helps a variety of non-readers learn how to read. There is probably a literacy program in your area.