Red Cross uses a lot of community service volunteers to do different things. Call your local chapter. Food banks and food pantries need people to stock shelves and clean up, Habitat for Humanity may have a building project going near you, the humane society may need dog walkers or people to help clean kennels, an environmental group may have a stream clean up or litter clean up going, the park service may need trail markers or trash pick up, a local church may need help with lawn work, call around and email and ask. You may need to work in more than one project to get that many hours that fast. They will ask what the offense was and maybe what besides not going to jail you want to get from doing community service,
One suggestion get started ASAP and try to get the hours completed by the end of the 3rd week . Life has a way of throwing us curve balls and through no fault of your own something could happen that last week that kept you from being aloe to finish up at the last day. (car break down, illness of someone supervising the project, horrid weather, your own sickness). PO's are generally unsympathetic to any excuse.