I invited a few friends rounds for a few drinks and we had a lovely chatty evening. It was great. We looked back on our lives and friendships thus-far and talked of how we would meet and mix in the future - as well as about the wedding.
I saw horrible spectacles of raucous groups of stupidly dressed females rambling around York the other year and it wasn't an edifying spectacle. I would steer clear of themed booze-ups, personally.