Rain or shine, be sure to pack warm clothes. The calendar may say 'summer', but Cape Cod in September will have you convinced otherwise.
The weather forecast for this weekend varies depending on who you ask. Heather suggested what to pack, and Susan specifically encouraged us to prepare for rain. Regardless of rain or shine, you should remember to bring warm clothes. It may not sound like sensible advice for someone who's going to be in the blistering sun for six hours at a time, but it's the other 18 hours you need to consider. The evening activities and ceremonies are held outdoors, and Cape Cod in September can be quite cool. Pack a sweater or two so that you can always dress down a layer if need be.
Oh, and for those ceremonies? You may want to bring a box of tissues or two as well.