A jacket. Cream or pale grey? Definitely not a sparkly cardigan, I think a cardie would look weird over a long-sleeved dress. (also, IMO, sparkly cardigans are a bit "granny at the Mecca bingo", but others may disagree).
Also, with the dress being long sleeved, you probably won't need anything over it while you're indoors. With it getting into winter, you might want something more substantial over the top if there's going to be lots of hanging around outdoors for photos etc. Do you have a smart winter coat that would go?
Then, indoors, I'm thinking just the dress, some nice jewellery and shoes/bag?