My favorite actions with gimp
-filters, edge detect, neon
-filters, map, bump map
-of course smudge
-free select, colorize, so there's cool colored orbs or something
I also usually turn the contrast up on my renders it looks cool
ps. Don't colorize the render(in my opinion) also cis said to use a small font near the render wich I think looks good. Especially when it KIND OF blends into the backround