Cancún is not a place where you will get bored, unless that is the purpose of your visit.  In Cancún even relaxing is never boring. There is plenty to keep everyone occupied when they want to get away from the beach for a while.  Explore some of Cancún's local Mayan ruins.  Go shopping, ride a bike on some of Mexico's most well groomed bike paths.  Take in a bull fight (Wednesday afternoons), go ride an ultra fast go-kart or experience The Rocket, if you are really brave.  You can rent a motorcycle or buggy and go exploring in the jungle, or visit the alligator farm or the nearby botany park. The local malls and restaurants could keep you busy for an entire vacation.

Water sports.
Cancún is a natural haven for water sports of all kinds.  Just about every kind of water sport is available here.  Almost every beach has vendors that offer parasailing, banana boat rides, jet skis and other varieties of small watercraft.  Parasailing is offered in different forms for one or two flyers, one of the variations offered here resembles a small flying boat that is perfect for the less adventurous.  Jet skis, waverunners and parasailing are available on the Caribbean and on the usually calm surface of Laguna Nichupté, which also offers near perfect conditions for water skiing.  World class scuba diving is available at many nearby locations, most notably, the islands of Cozumel and Isla Mujeres.  A wide variety of packages are available, most of which include transportation.  Snorkelling is practiced everywhere but the best locations are generally away from Cancún proper.  A modern  water park with a wave making machine, rivers, slides, a free fall thrill ride and restaurants, is located near Punta Nizúc.

Sport fishing.
Cancún and the surrounding area offer some excellent sport fishing, deep sea fishing in the outer Caribbean for sailfish, marlin, dorado, wahoo, grouper and blue fin tuna.  The many nearby inlets, flats and lagoons give anglers a chance to try their luck at catching tarpon, snook, permit and some hard fighting bonefish.
Charters can be arranged through the activity desk at your  hotel or at one of the many marinas along Laguna Nichupté.  Nearby, Isla Cozumel also has a small, though well equipped marina and a reputable sport fishing fleet.  If you happen to be spending a few days on Cozumel the fishing grounds are a little closer to the island and therefore you get more real fishing time for your money.

Diving.
If diving or snorkelling is on your agenda you have certainly come to the right place.  The waters of the Caribbean are noted for their clarity.  Some of the world’s most famous reefs are just a short distance from your hotel.  There are underwater national parks and plenty of dive sites that are easily accessible from many of the nearby beaches or islands.

Golf.
If you enjoy golf, you will not be disappointed with Cancún.  There are two championship 18 hole golf courses located in Cancún proper.  There are also two 3 par executive courses and a miniature golf course for the kids (and grownups).   There are two more 18-hole courses less than an hours drive away.  One is at Playacar, next to Playa del Carmen, and another is located at Puerto Aventuras a little farther down the coast.

Tennis.
Tennis can be enjoyed at most of Cancún's hotels.  Many will allow non-guests to play, although, sometimes a small fee may be payable. Pok-Ta-Pok Golf Club has two tennis courts that are open to the public. They also have a clubhouse with a waterfront restaurant that is in a very pleasant location, away from the crowds.