There is the Botanical Garden, a very beautiful and peaceful place to check out.
Then there is the Faro A Colon, a huge building that is very impressive when seen in person. Get there before it closes around 5 or 6 I believe so you can tour the inside.
http://www.el-bohio.com/faro/photos/faro7.jpg
The aquarium was a bit disappointing, I wound up feeling sorry for the poor fish that are forced to call that place home so you may want to skip it.
And there is the series of underwater caves and lagoons in Parque Los Tres Ojos.
http://img261.imageshack.us/img261/5...viercarcl7.jpg
You go down some steps and take a boat that separates the underwater caves from the lagoons. It is quite pleasant.