Shut down all power to vacant offices overnight, including unused computers, printers, phones, servers, network. Stop the drain of "wake up" state.
Many employees in my building have mini refridgerators, coffee makers, and microwaves in their cubicles (aside from the main appliances in the office kitchen). Estimates of mini fridge cost alone range from $30-$75 per year, not including other appliance cost and the amount of employees using said appliances. Each office could save hundreds if not thousands a year by using community appliances.
Make it automatic that overnight computer monitors (but not the computers) are switched off to conserve electricity, thus saving money and reducing carbon footprint, impact on electricity grid, etc. For those working at night, this could be easily overridden. Walking down the hall at my building I can see enormous potential for savings with this.