UKC

Is there any DIY accessories, gimmicks or other things that can be done to a retired harness?

Turned my first (Wild Country) harness into a belt for my son, I’ll stick a photo of it on my profile

You can easily turn the waist belts into belts, if you carefully unpick the webbing from the foam/padding. 

Unless it's damaged, keep it in case you ever need to do some extreme DIY such as dangling from your roof to repair a drainpipe, etc.
Similarly, it could be used if you ever need to bolt routes and don't want to trash your good harness.

Use my old one for minor tree surgery - no falls involved, just needs to prevent them

In our family the climbers pass their old harnesses and other gear on to the sailors. It’s going to be better than marine tethers that unclip themselves at sea because there are no thorough standards and nobody has thought about cross loading, and mast ascent gear that is an old splintered plank roughly tied on to some braided rope that has spent its life out in the sun and is covered in salt crystals and nasty oils.