It is always a good idea to have a separate, open workspace away from office desks and filing cabinets where employees may work away from their desks.  A separate workspace encourages workers to meet and discuss their projects; an area which will not disturb their other colleagues and has its own office chairs and desks equipped for meetings.

Your building ventilation systems, such as air conditioning, must be checked regularly.  Keeping fresh air regulated throughout the office has shown that this helps prevent feelings of lethargy.

Large windows, particularly near the working area, around office desks can uplift employees, as studies show that natural light can brighten people up.  Failing this, up-lighting is a good idea to brighten up the entire workspace. 

Allowing employees to bring some of their own personal items, e.g. plants or pictures to brighten up computer desks and filing cabinets brings some personality to the office and makes workers feel more comfortable.  A few trinkets or family pictures on office desks can make all the difference.