That's a tough one. What kind of job is this for? Sounds like a trick to me. I would avoid being "VERY casual" and go with some kahkis, a nice polo and decent shoes. That way, you look more dressed up than usual but not in a full out suit.
This is pretty for a casual look. The link is teh onecalled silver dor belted skimp dress. cute! For the wedding one... hmm Hope you like em! good luck!