I used to volunteer for the Lost Dog and Cat Foundation. They hold adoption events every Saturday at the Falls Church PetSmart and need volunteers to handle dogs and cats. You just sign up for their email list on their website and then every week they send out an email asking for volunteers that weekend. If you want to volunteer you reply to get a spot. So you can choose when you volunteer. Everyone I have dealt with there (I ended up adopting a dog a few years ago from them) is extremely passionate and I never had a bad experience.