A cute planted shrimp tank would look gorgeous in that tank. The size is not really big enough for fish but dwarf shrimp are active colorful and will often breed in the tank which is a lot of fun to see.
Ghosts are fun but not very colorful and theyíre not always the healthiest.
I would do a species of neocaridina (like the cherry shrimp). Theyíre available in all sorts of colors (blue, yellow, green, red, chocolate, striped.). You can mix colors if you want but if they breed you may find offspring reverting to less vivid colors. Most people pick one color per tank.