Water
We shouldn't have to mention that shipping and wrapping water in plastic (which still leaches chemicals, even if you avoid BPA) is a bad idea. A great green step for conferences -- simple and off your budget -- is to let attendees know whether the local water needs to be filtered or not. Especially if your conference has a similar group of attendees each year but moves from city to city, talking about where the local water comes from and how clean the local ecosystem is creates local, memorable connections between your attendees and the ecosystem surrounding the walls of your conference center.