This may sound like an unusual ministry but it is actually quite effective. Many homeless people take in dogs or puppies and care for them. Doing something kind for a homeless persons dog is the same as doing something kind for them. Their dogs are usually around plenty of people and are generally friendly. Dog treats, dog food and flea drops are great things to give. Just keep a supply in your trunk and visit the camps every week or so. Get to know the dogs and the people. After a few weeks the people and their pets will be happy to see you.
If you know a veterinarian that does not mind volunteering some time you can have them come along now and then to give shots. If not then you can just recommend one that is close by.
As you minister to pets you will have the opportunity to talk with the people about their situation and therefore minister to them in other ways.