if your going to add neons go all out and do cardinal tetras.
i would stick with the muds. get some of those waxy twiggy branches that people put with birs or reptiles and have that be the roots of a mangroove and arrange it half in half out with the water and sand substrate. stick some fiddler crabs in there.
or for a playful one, do a beach with like a carribean background and out little fake beach chairs and mini cooler and a parasol in there, i don't know kinda' kitchy