I did centerpieces for a tropical themed wedding once.
I used pieces of drift wood on the tables. I then took a couple small dixie cups, covered them in moss, places wet floral foam in them & tucked a couple tropical flowers into the foam in the cups, real short, just above the lip of the cup, then placed the cups w/i the drift wood. You can add a few sprigs of monkey or bear grass for hight. Place a few shells on or near the flowers/driftwood. Cheap, easy, pretty.
Flower example 1 open lily head cut real short & 1 spriggy orchid in a cup.
Another easy tropical decoration would be to trail ivy or some other trailing folage up the center (zig zag free-style) of the buffet table (or other table) & scatter coconuts, pineapples, shells, etc along the folage, add a few loose flower heads/blooms that match the flowers in the center pieces.
You can stick same themed flowers in & around the cake in a random pattern or create a waterfall off the top & running down the side. It is OK to stick the stems straight into the cake.
Be sure to elevetae the cake on something, anything! You can use a short box covered in a table cloth, just be sure to elevate it, a flat cake looks cheap.
How about messages in bottles on tables, you can make scrolls that say so & so loves so& so & put a little sand & shells inside, make sure you roll the messages to face outward.
You can use tiny messsage in a bottle as either favors or place card holders.
More later.