We did the swim, push, pull swim. It was great . Afterwards you can buy a t-shirt in their LARGE gift shop of the dolphin you swam with (about $20-$24) and video and pictures. They have a big pool and swim up bar. several rest. to eat at. And they have the Mayan ruins you can explore for free.
You can also pay for Snuba. We heard the current was a bit strong but it was in November 2008 we went. Should be better in March.