To be honest alot of my favourites have already been covered.
But to add to the ones mentioned:
Normans lampeye killifish are a nice little fish and unlike most small shoalers they're quite brave and will happily swim at the mid-upper levels.
Maybe pair them with something like the badis Cazgar mentioned, or a solo male apisto (but on of the smallest varieties) or a pair of female apistos from the smaller varieties.
There are also some gobies and killifish that might be suitable.
Not to mention sparkling gouramis/dwarf croaking gouramis. They're very timid aswell so great for small species tanks. With dense cover and no other fish they're really lovely to look at. Not particularly active though...