Isla Contoy is a bird sanctuary off the northeast corner of the Yucatan peninsula. There are no hotels or permanent residents there, but one can easily make half day trips by boat from either Isla Mujeres or Cancun. The only buildings on the island are a ranger station, an observation tower, a small educational display, and some picnic shelters. Because people don’t live here (besides maybe a few park rangers and scientists) and the number of visitors is limited, the wildlife density (particularly birds, but also lizards, crabs, rays, etc.) is tremendous.
Because they try to limit where people go to avoid interference with the wildlife, a few hours here is sufficient. However, there is a pristine white sand beach that, as I indicated, is visited by rays and fish.