You could do black dresses (most everyone already owns one).
If sitting in the floor is an issue the best bet would probably be black pants or dark wash jeans, paired with heels and a similar colored top. ex: dark wash skinny jeans, either black or nude heels, cream or off-white top. To make it more informal, you could even not specify the color, just make it no prints or something.
Also: Cardigans are super cheap at target, you might be able to use those as an option.
Threaded Mode
