Mandate that cabs can't be more than 4 years old, and do what NYC just did: require that all cabs become hybrids within 4 years. The cabs in Boston are shite compared to any other city I've been in. Get rid of the asinine rules about a Boston cab being banned from picking up Cambridge passengers and vice versa.
Put newspaper recycling bins in the subways, and have automated announcements and signs telling people that leaving one's Metro in the subway is littering. This is a pet peeve of mine. I once pointed out to a young woman, who had just dropped her Metro on the rain-slick floor of a Red Line car, that she dropped her paper. "I meant to," she replied. "That's littering," I scolded. "No it's not," she countered, "I put it there so someone else can read it." This young woman in all other respects looked normal--she didn't smell, was dressed properly, wasn't twitchy; but nonetheless, somehow her parents raised a fucking idiot. Rant over.