San Gervasio and the mainland sites are two different days, and it helps to know which one you're signing up for. At San Gervasio you walk shaded jungle paths between low temples and platforms. It's quiet, it's quick, and it stays on the island, so you can be back at a beach club by lunch. The trade-off is scale: this was a pilgrimage site for Ixchel, not a monumental city, so the structures are modest and an hour is plenty. The mainland is the opposite trip. Tulum hands you a walled city on a cliff over the Caribbean, and Chichen Itza delivers El Castillo, the towering pyramid. The catch is the travel. Both start with the ferry from Cozumel to Playa del Carmen, then a drive that runs from roughly an hour for Tulum to several hours each way for Chichen Itza. Add the ruins on top and you're looking at a full day, most of it on the road and on your feet in the heat. If you go mainland, bring water, sun protection, and shoes you can walk all day in, and go in knowing the ruins are the payoff at the end of a long travel day.