How about a wicked trouser suit? It will cope with the inevitable weather issues - and you'll look a bit different from the usual crew of dress'n'jacket guests. Go for something with a punky edge (have a look at the trouser suits at YSL, Karl Lagerfeld and Helmut Lang) - and give it a feminine lift with vertiginous heels in coloured leather, a great clutch bag (snakeskin? metallic?) and a corsage on the lapel.