For a casual date I usually go with either jeans or pants, a blouse, and a jacket or coat of some sort with some wedges maybe. I wouldn't wear a dress only because I only wear dresses when I dress up and would feel overdressed. But a flow top and tight pants might do that hide/flaunt thing you're going for!
If you want more to hide behind, you can try wearing an oversize sweater or sweater dress and leggings, and some comfy boots. Maybe you could put your hair up in a curled ponytail, and add a cute barrette. :)