Can you do anything worthwhile with just one flash?

Sure you can. This, of Sheridan student Steph, is a single flash, off camera, without softbox or umbrella, but fitted with a 1/4″ Honl Photo grid::

1/125 sec, f/8, 100 ISO (the studio settings); flash at 1/4 power with grid.

Learn to take your flash off camera and have fun!