I suppose depending on what continuity you're in, your Captain America costume is going to need a mask like the Deluxe Captain America Full Vinyl Mask. Of course even if Steve Rogers' identity as the first Avenger isn't know, it's still a cool addition to your costume this Halloween (or comic con or whatever).
Just look at this baby. Add it onto any costume you've got. From just a t-shirt or you might be going as crazy President Red Skull from Old Man Logan. I don't know. I won't tell you how to dress up with the Deluxe Captain America Full Vinyl Mask! The world is yours!