The airport is across the harbor from the cruise terminal and from most of Boston. There isn't really anything "near" the airport to hang out that is going to be particularly easier to get to the airport from than elsewhere in Boston, so I'd pick a place in town, like Copley Square, as suggested, or near Fanueil Hall or along the waterfront, depending on the weather. If you want to stay close to the Cruise terminal and only "transfer" once, you could probably walk to the Institute of Contemporary Art along the waterfront, which has tons of seating and great people watching, plus options for a quick meal.