The rules and regulations in USA are such that it not easy for foreigners to be able to work in America. To work legally you will need the AJ1 visa that is given to university students for vacation employment, carrying on work at summer camps, and short-term traineeships (volunteer work) with a specified employer. If you volunteer through a reputable charity organization and are not paid for your services, they will even guide you in how to apply for this visa.
One option for volunteer in USA is working in the national parks. Program durations can be 8 - 12 weeks or you can select the shorter duration program of twelve days. Volunteers help with maintenance and repairing of fences that protect the species in danger, re-vegetating, and building trails.
It is easy to locate programs that suit individual tastes by visiting the government volunteering sites. By putting in information like location of interest, type of project and duration, you can get a list of all the projects matching the entered parameters. If you go through the official sites, and select a government approved project, getting a visa becomes that much easier.
For people who enjoy working with animals there are animal shelters that require volunteers. Depending on personal taste you can work behind the scene in administrative work, or spend time with the animals by caring for them and feeding them.
The variety of volunteer options is almost limitless and if you work with a recognized charity even the visa is not a problem. All that is needed is the willingness to actually go out and do the work.