On the similar theme as cloth library bags for sale, why not try selling reusable coffee mugs? I know some libraries’ friends groups sell travel type coffee mugs w/ the library logo, raise money, and save the waste of throwing away paper/Styrofoam cups. Did you know switching to a reusable mug for just one day will save as much energy as using 1,000 gallons of gasoline (besides saving wood, carbon dioxide, water, etc) Many coffee places also give a discount to users who bring their own mug.
If you have a coffee shop/station in your library, think about using fair trade, organic coffee as well…. many colleges across the county are getting on board with this idea already due to rallying college students.