get 80 watts pc or more and your good to go with a wide selection of corals.
gobies are great for small tanks, depending on the goby of course. mandarins are not gobys but dragonets.
some clownfish can live in 12 gallon tanks.
alot of damsels are available and do good in small tanks, multiples of less agressive species or 1 of the much more agressive kinds.
royal gramma does alright in the nano tanks.
firefish, firegoby, do good in any tank, but i recomend a lid for these guys, as i found my guy dried up behind the tank...
many interesting inverts that do well in small tanks, starfish are generaly not a great idea though. neither are flame scallops.
most marine fish can become agressive and fight for space, best to have alot of live rock and hiding space and most importantly IMO is to add all fish at the same time OR rearrange the rocks each time a new fish is added. (i almost lost my royal gramma due to this mistake, blue damsel and false perc can get nasty!)