What are the best day trips from Cancun?

Isla Mujeres over everything else if you have one free day together. The Ultramar ferry leaves Puerto Juárez every 30 minutes starting at 5:30am, takes about 20 minutes, and costs roughly 300 MXN round trip. You land on a 7-km island where the north end is a wide, calm beach called Playa Norte and the south end is a rocky point with 20-meter cliffs. Rent a golf cart for about 500 MXN and you can cover the whole island by lunch. One of you can float in the warm, chest-deep water at Playa Norte while the other walks the sculpture garden at Punta Sur. Meet at Playa Lancheros for tikinxic, a whole fish rubbed in brick-red achiote paste and slow-grilled over coconut husks. That fish runs 250-350 MXN. The cold Montejo Especial next to it costs 45 MXN. You will be back in Cancún by 5pm.

Tulum is 130 km south, and the ADO bus from the downtown terminal takes about 2 hours each way for around 200-280 MXN one way. Leave on the 7am bus or you will arrive at the ruins after the 10am wave of tour buses from the Hotel Zone. The ruins sit on a limestone bluff above a small cove, and the view from El Castillo down to the turquoise water is worth the early alarm. Admission runs about 100 MXN. The path down to the beach below the cliff is steep and sandy, and the water tends to have a moderate current. If one of you wants hours at the ruins reading every plaque and the other wants to swim, Tulum is one of the few Maya sites where the beach is a five-minute walk from the temples. Grab fish tacos at the stands near the parking lot for 60-80 MXN each before catching the return bus.

Valladolid is the sleeper pick but works best as an overnight. The ADO bus takes about 2.5 hours and costs 250-350 MXN one way. The town centers on a colonial plaza where you can eat papadzules, corn tortillas rolled around hard-boiled eggs and covered in pumpkin-seed sauce, for under 120 MXN. Cenote Zací sits inside the town itself, a limestone sinkhole with a staircase carved down to dark green water. Chichén Itzá is only 40 minutes west by colectivo from Valladolid, but trying to do Cancún to Chichén Itzá and back in a single day means five or six hours on buses and roughly two hours at the site. Stay one night in Valladolid for 600-900 MXN at a guesthouse near the plaza, visit Chichén Itzá the next morning when it opens at 8am, and bus back to Cancún after lunch.