Agree with susieblue - you need to know if the day guests have been asked to glam up, that will set the tone of the evening.

If you can wear your trousers, which sound lovely, then why not a nice kaftan style top perhaps with some beading around the neck. I love this type of look and it doesn't matter what's going on underneath!! A nice pair of sparkly sandals, pretty little bag and some great earrings and you'll be good to go.