My bonus room over the garage is always cold.  Is there anything that I can do to make this room more comfortable?

Airflow and heat loss are the most common reasons to have uncomfortable areas in your home. The first thing is to check air flow to your heat registers. If there is no airflow, remove the register and check for any physical blockage. If no blockage is found it may be that the ducting has come apart or the ducting has been undersized with too many elbows.

If the airflow is fine, it may be that your garage is cold. Heat energy flows from a hot area to a cold area. If your garage is cold, your floor in your bonus room will be cold. The heat will drawn thorough the floor into the garage making your bonus room uncomfortable.

This can be rectified by either installing a garage heater to maintain a temperature of 10 degrees, by creating a thermo break between the garage ceiling and the bonus room floor. Or by installing a zone control system which will allow you to have total control in all areas of your home. For this you should call a professional to look at it as there will be a number of items to assess prior to any repairs.