My daughter requested a pink treasure chest birthday cake for her birthday. I used an 11 x 15 cake pan and 2 batches of batter. After the cake had cooled, I cut the cake in half and frosted/stacked it. The lid was molded rice crispie treats covered in tinted butter cream frosting.
I was determined to make the entire cake completely edible, but the lid started cracking, so I would advise putting a cardboard bottom to the lid and then placing the rice crispie treat lid on top to add more stability.
The decorations were made out of chocolate plastic (roses/straps/lock) and molded white chocolate with gold edible spray paint for the coins.
I found rock chocolate candy, shell-looking muffins, candy necklaces and silver candy balls to add to the look.
The base was covered with crushed cookies for that washed-up-on-the-beach look. It was a fun cake to make, but it was REALLY BIG!