Donate wedding flowers to those who will really appreciate them, perhaps to a nursing home, womens' shelter, or any other organization that could use some brightening up.
If a RSVP is requested on an invitation, let the host or hostess know whether you are going to attend; it's an easy thing to do and is a common courtesy you should extend to anyone inviting you to an event.
Sending a thank you card is a small gesture that goes a long way. Expressing your thanks with a quick note will mean a lot to the person who did something nice for you.