Sure, Cancun is great, but no matter how opulent the pool is that you're lazing around, it can get monotonous after awhile. Here are some fun day trips to get you out and moving to explore everything the Yucatán has to offer.
This Mayan city is one of the best-restored archaeological sites in the country. It's also conveniently located just a couple hours away from Cancun. This is a pretty significant tourist site, with 1.2 million visitors, so the price for entry is a little higher at around $14. You can go with a group if you want—most resorts offer day trips or can at least point you to a tour company—but you can also head down on your own, pay a small parking fee, and hire a guide you like onsite. Things to do include El Castillo, a huge stone pyramid with the head of Kukulkan, the serpent god at the base. On the equinox, the sunlight casts shadows that make it look like Kukulkan is slithering down the pyramid. Go between October and April for the best weather, but be aware that it's also high tide for tourism.
The remains of this Mayan seaport are located less than two hours south of Cancun, down Highway 307. Entry is a pittance—less than $4 a person—although you will wind up paying more if you go as part of a tour group. The ruins have a lot to offer, from the incredible limestone wall that serves as a defensive fortification, to the clifftop castle with its view of the Caribbean. If you go between October and December, you'll get the best weather and the least amount of fellow tourists.
Xel-Há Park is an adventure park located just north of Tulum, about an hour and a half down the same highway you use to go from Cancun to the Tulum ruins. Admission is $89, but children's admission is half-price, and you can get up to 15% off if you book online far enough in advance. You can snorkel among the waters of the "natural aquarium" along the Riviera Maya, explore Xel-Há cave, or try out any number of zip-line courses. Bring beachwear and extra clothes, and make sure your sunscreen is chemical-free, or they'll hold it for the day while you use theirs.
Isla Mujeres is about an hour out from Cancun. You'll drive up to Villa del Palmar, and from there you take a ferry out to the island for 80 pesos—less than five bucks. (Though if you want to do it like the upper crust, you can always fly in.) Once you're there, you can snorkel the coral reef or head out to the turtle farm to check out baby turtles. Playa Norte is an incredible beach, and if you're not tired of history lessons, check out the Hacienda Mundaca, built by the pirate Fermin Mundaca. The ferries start at 5:00 a.m., and the last one heads out at midnight. By then, the crowds are usually thinner, so you can have more of the beach to yourself.
Also known as the Cenote Trail, this road takes you through eight naturally-occurring sinkhole pools. There's a lot to see here, and each one is its own attraction, capable of keeping you busy for several hours. From snorkeling in caves to zip-lining over the water, these natural pools have a lot to offer. Boca del Puma is a polished experience with zip-lines and ATVs, offering both for around $100 per person. There's also Kin Ha which offers zip-lining and ATVs, as well as cave diving.