Centerpieces are one of the most popular DIY areas for weddings.
Keep in mind though that creating centerpieces can be VERY time consuming, and that to keep flowers looking fresh the centerpieces are best done the morning of the wedding, or the night before (if they can be stored in a refrigerator). If you don't have a lot of willing helpers for the morning of your wedding, or an industrial size refrigerator in which to store the flowers overnight, you should consider a professional florist, or non-floral centerpieces.