Bring a plant - get a plant! (or 10, or 20, or 30...) You can also swap cuttings, seeds and seedlings! Ornamentals, vegatables... if you've got it you can swap it.  
Cost to participate is 2,000 colones, which will be donated to the Guardavidas Costa Ballena Lifeguards.
Let us know on the event wall on Facebook what you're thinking of bringing, or if there is anything specific you are looking for. And please ask, if you have any questions!