I visited Marathon Key, Florida and went to Sombrero Beach for the day. Sombrero Beach offers a beautiful white sandy beach with shady park with picnic pavilions, volleyball courts, restrooms and showers. The beach was located in a neighborhood so I thought I miss read the GPS but when I arrived, I was pleasantly surprised. The pavilion is very large with an abundance of tables and benches. We just couldn't see the beach from the pavilion because of the scrubs between the beach and the pavilion. Once on the beach, the palm trees and white sand were just a gorgeous sight.
If you're into conservation and helping sea animals, there are two centers that are great with raising awareness and rehab for hurt sea life. I spent some time here when I lived in Naples and they are great if you have some time to visit.