You'll be responsible for putting less styrofoam into the waste stream. Return others to the brochure stand. Don't buy endangered species products such as tortoise shell, ivory, animal skins or feathers. Don't purchase star fish or turtle-shell related souvenirs or any creature that may have been put to death for the sake of a gift shop sale. These animals may have been killed specifically for tourist trade.
Sea shells are questionable.
The U. S. Customs office offers a list of items which cannot be imported or brought into the country. Make a pledge to yourself not to maim or kill any living creature other than enemy or pest insects. Take only photographs. Leave only footprints. Take everything out that you brought with you.