My niece is having a destination wedding in Costa Rica which my husband nor I will be able to attend. Her sister had her wedding in Hawaii two years ago, which we also weren't able to attend. (I have two children in college... enough said!)For her sisters wedding I had arranged after the ceremony for them to set free a box of butterflies. It was a surprise for the bride and groom and all the guests except my sister who read a poem I had written. I was told it was absolutely beautiful and made the ceremony that much more special. I would like to do something for this wedding which is in April. Please, if anyone has any ideas, can you pass them along. I would like it to be just as lovely and special as the butterflies. Thank you all.

I take it you don't want to do the butterflies again...
Flowers are what I think of, maybe some little Costa Rican girls with garlands, singing, and some live music...

BUT
I am almost obsessive about gifts that have no impositions, no requirements, no strings attached. This after years of familial and in-law uncomfortable gift situations. And I'm a big fan of Miss Manners.
I would be worried that a bride (or groom, or bride's mother, anyone) might feel that an addition to the ceremony deflates what they had planned, or is simply not what they like, or it makes for a captive audience and they don't want that.
I might have flowers delivered to their room every day for a week or something. It's a gift they can like or not in private and do what they want with it.
Or even more likely, I would send a present to their home. You don't need to give gifts at the wedding.