Groom's Ensemble
There are plenty of alternatives more interesting than a tuxedo.

Matron of Honor/Bridesmaids
It's hard to beat a standard bridesmaid's dress for sheer terror, but here are some other options.

Best Man/Groomsmen
Suggestions for the male attendants.

Officiant
Depending upon who the person performing the ceremony is, they may want a costume as well.

Guests' Costumes
If you want your guests to "get into the spirit," try these suggestions.