Whatever you pick out, it'll be fine.
I'd say a cocktail dress. Pearls. Non-stiletto shoes. A purse that looks like a purse and not like a lunchpail. You know.
Non-stiletto shoes?
Stilettos are mandatory with cocktail dresses.
BEG....I'd go with black gabardines or velvet slacks or a black pencil skirt...soft dressy sweater or ruffly blouse in red, deep purple, black or cream, maybe with a satiny camisole underneath...definitely high heels with either the slacks or skirt. Go middle of the road....that way you aren't under or overdressed. If you talk to the wife of the immediate boss....ask her what she is wearing...
Don't know what type of jewelry you prefer....I always wear diamond studs, and I have a diamond pendant that I wear most of the time....as far as I'm concerned...diamonds always are appropriate.