You can choose different arrangements of flowers, candles, vases, fruits etc. (even favors can be nice centerpieces). Visit IdeasForCenterpieces for more ideas.
Candles are a good formal alternative to flowers. Pictures of the bride and groom as children, which can be arranged in flowers. Depending on your favors, you can also use these as centerpieces.
The most unique centerpieces center around something meaningful to the couple. Follow your occupation. If you're both CPAs, creative use of ledger paper as vases give a unique look.
I would suggest buying some vases and adding either fruit or silk flowers and water to them. On top of that I would put floating candles. That would be very romantic, but yet fun and different. Also you could get some branches, put them in a vase, and hang crystals from them.
