Maxi's are always a fave - they can be dressed up or down to suit the venue.
Otherwise, go with your instincts! If you have a dress you love, wear it and dress it up and down depending on the style! I have a full-length black one-shouldered Grecian dress with a sequin band under the bust that I just love. I always feel its a little dressy, so I wear flat sandals and more 'casual' jewellery (bracelets and hoop earrings) and it brings it down just enough
Anyhoo, good luck and if you're still not sure - post pics!!