For something like this I'd probably try bouncing it backwards and using the whole back wall as a diffuser. This confuses metering, so sometimes you have to set the exposure manually.

You can also buy clip-on diffusers which work fairly well and remove some of the hard edges, stark shadows and flattening.

Hand held flash can also work. Close the shutter as far as it goes, open it with a bulb setting for a second or two to add some ambience and movement blur, and manually fire the flash off the back wall.

Buying a crossbar support for the flash to move it off the lens axis also makes a bit of a difference. As a last resort you can use loo paper as a very cheap diffuser, but that's another good way to confuse the exposure metering.