If you are interested in wildlife, you will enjoy exploring Celestun.

The wildlife reserve at Celestun is a bird sanctuary best known as a flamingo breeding spot, as well as a stop-over for migrating North American birds.  Depending on the season, you will be able to see cormorants, egrets, storks, pelicans, ducks, etc.  Although it is very likely that you will be able to watch flamingoes, it cannot be guaranteed.    There are two options:

The traditional Celestun visit

Consisting of a one or two hour boat ride along the lagoon / tidal inlet, birdwatching at several spots, navigating through a mangrove canal and swimming at a fresh water source.    The two-hour boat ride includes a short stop at a petrified forest plus navigation to the spot where the estuary meets the ocean. 

Dzinitun

If your are more into physical activity, the best option is Dzinitun (within Celestun).  This site is oriented towards eco-tourism and land / species conservation.  The basic plan takes about 3 hours, and you will have ample time to explore the area: ride along a magnificent mangrove canal on an electric-propelled boat (to avoid water pollution) or take a kayak instead.  There are long stretches of trails along the wetlands and numerous bird watching posts.  For the last leg of the trip (1 km) , ride a mountain bike along flamingoe feeding grounds.    Lunch available at Dzinitun (fish & seafood).

After the visit, you can continue to the coastal town of Celestun to enjoy the beach and/or have lunch at one of the seaside restaurants