Depending on where you are having your ceremony, you may need to cater seating for your guests. Ask your Wedding Planner for a few more chairs, always consider pregnant ladies and the elderly.
If its an outdoor wedding, they will need water, chairs and shade. Why not include a drink station at your ceremony, you don't want anyone stealing your thunder and fainting before you say 'i do'.
Your plan B - what if Mother Nature doesn't play fair? You may need to consider your options. Speak to your wedding planner about how you can ensure your ceremony stays as gorgeous as your dream even if it doesn't happen to rain.
Don't forget, if you end up going with plan B, your photos will be incredible, don't be disheartened - it will still be the best day of your life to date.