The dolphin swim indeed is quite short and also one needs to be aware of the differences in the different dolphin programs, whether it is swim, interaction, encounter, etc...on some you get actually to swim with and be pùshed by dolphins, in others you may only touch them. Otherwise, Xcaret has A LOT to offer other than just sitting on the beach, there are many attractions, activities, expositions, etc...a great Eco-Park!
For shopping, the 5th Avenue of Playa del Carmen is highly recommendable. Calica offers close to no shopping but Playa dC. is only a 10 minute taxi ride away from the port...you will find ANYTHING there!
Also a lot of adventure tours are available such as ATV, caverns, cenote snorkel etc...good luck!