The pier is situated almost half a mile away from town. There are the typical tourist shops on the pier along with the duty free store. Although a shuttle is available, access to town is limited and inconvenient. Progreso itself is very small with a nice beach. The nearest larger town is Merida which is rather nondescript and a bit of a disappointment. Nearby Mayan ruins are interesting but incomparable to Chichen Itza.
Carnival's pier is located nearly 8 km. from the centro district. The entire pier is a Carnival built and sponsored mall of tourist shops and duty free store. To get the real deals, you'll have to leave this area and go into town. Across from the pier are independent shops and scooter rentals. We rented a scooter with gas for US$25. Be sure to get helmets. I don't recommend the scooters unless you've driven in third-world countries before and are used to traffic chaos.
The entire island can be traversed in about 1.5 hours. Start early. Traffic will slow you down in town. A great spot to check out is at the southern tip of the island. A park named Faro Celarain offers a lighthouse, beaches and snorkeling for US$12/person. A great place to get away from the cruise crowds.