For fun, what would you wear to a black tie optional wedding in the late spring?

I'm starting to look for my BIL's wedding coming up in a few months. I'm not in the wedding but dh and my older daughter are. Bridesmaid dresses are long and formal, so I figure I should dress on the more formal side. And I haven't bought a fancy dress in forever! Finding something that won't make me look pregnant and that I can wear a real bra with is tricky!

ETA: pictures or links if you have them!