15. Dry Tortugas National Park, Florida
Source: Iconosquare
The only way to reach this island is by boat or seaplane. Spend the day enjoying the abundant sea life or watching the multitude of colorful tropical birds that migrate to the island for breeding. Picnic near Fort Jefferson, the largest masonry structure in the Western Hemisphere. This coastal wonder was left unfinished, but still boasts sixteen million bricks.