Trips worth making
Cozumel is an island, so every day trip starts with a 45-minute ferry back to the mainland from San Miguel to Playa del Carmen. That crossing eats a real chunk of the day, and honestly the best trip from Cozumel is usually another dive on the reefs you came for. But if you have the days, a few mainland runs earn the ferry ride. Ordered by value.
Playa del Carmen — about 45 minutes by ferry
The easiest hop, since the boat lands you right on the pier. Playa has a long walkable beach, the Quinta Avenida pedestrian strip, and far more restaurants and nightlife than Cozumel. Good for a change of pace and a shopping-and-dinner evening off the island. Worth it if you want a busier night out; skip it if you came specifically for quiet water and reef, because Playa is louder and more commercial than the island.
Cenotes near Playa and Tulum — from about 1 hour off the ferry
The mainland’s freshwater cenotes, swimming and snorkeling in clear limestone caverns and caves, are the sharpest possible contrast to reef diving, cold fresh water and stone instead of coral and current. Cenote Dos Ojos and the string of cenotes along the Tulum road are the accessible picks, and certified divers can do cavern dives here. Worth it if you have an extra day and want something genuinely different from the boat.
Tulum ruins and beach — about 45 minutes ferry, then an hour-plus drive
Cliff-top Maya ruins standing over the Caribbean, plus Tulum town’s beach-and-restaurant scene. The ruins themselves are genuinely worth seeing, but it is a long day from the island, and Tulum has grown crowded and expensive. Go early to beat the heat and the tour buses off the cruise ships. Worth it as a full day, not a rushed half; better still as an overnight rather than a same-day round trip.
Cancun — about 45 minutes ferry, then 1 hour by bus
Big-resort beaches, malls and an international airport. Mostly useful if you are connecting a flight through Cancun or want a city fix. As a leisure day trip from a dive island, it is the weakest of these. Skip unless you have a practical reason to be there.
Honest take
Do not carve a mainland day trip out of a three-day diving trip; you will resent the lost water time. Save the excursions for after you have had your fill of the reef, or better, add a night in Playa, Tulum or Bacalar at the end rather than doing it as a rushed there-and-back. All travel times are approximate.