Hmmmm,
Although its a new tank, you could add a conch to it, provided you target feed it. They are really interesting to watch and will work wonders on your sandbed. I did this with mine and its doing fine.
You could try one of the more colourful pistol shrimp species and later pair it with a goby. You could maybe go for some of the more colourful snails like the bubblebee vibex, or some of the astreas have really beautiful shells (spiny star), nerites are also pretty nice.
Mithrax crabs are cool too, although a bit reclusive. They do a great job on algae. Mexican red leg hermits are very pretty and if you look for some fancy shells on ebay and add them to the tank, they will try them on for size.
I seem to remember reading somewhere recently that pin cussion / tuxedo urchins are also good for nano tanks, although i'd check this out yourself first as my memory isn't the greatest lol.