Is there a way to hide ONLY the column guides in inDesign, leaving the ruler guides and margins visible?
I don't think you can toggle the visibility of margins and columns separately. Either both are on, or both of them are off.
One workaround is to manually add the column lines by drawing the lines on a Master Page on a new layer. When you go back to your pages, you can then toggle the visibility of that column layer.
Every page with this Master Page applied will get the column guides.
If you simply want NO columns -- essentially just one big column, you select the page or master thumbnail on the Pages window and then from the Layout dropdown menu, choose Margins and Columns and then make columns be 1. If you just want to temporarily hide the columns guides, it's the answer the previous person gave.