I buy a big red bow at a craft store. You could easily get ribbon and make one, too. I secure the bow with a velcro strip at the top of the column. Of course you could simply tie the bow onto the harp. My harp is ebony so red really pops and I like the bow to just be in the front. The velcro I use is black which does not show on my harp. Velcro now comes in a tan color, too. I would suggest finding ribbon that works with your harp and the decor of the restaurant. You can also pin or glue a decoration in the middle of the bow. One year I clipped on a red sequin bird for extra pizzaz.
Another suggestion is to wrap garland on the column. You could add battery lights, too.