You must snorkel/scuba in Chankanaab Park. Swimming with dolphins is an amazing experience for people of all ages. On the other side of the island, the ruins are a must see -- amazing -- and the animals living there are extraordinary. Also on the other side of the island there is La Playa Bonita (the Beautiful Beach) -- there is no snorkeling there, but surfing and the beach is not crowded.
Quick Tips:
Cozumel accepts US currency, which is most common, and pesos as well, which is not so common as in other cities in Mexico.
Cozumel is also the one of the best places in the world to buy jewelry, but you learn that when you arrive. A tip for buying jewelry in Cozumel: go to the town when a cruise arrives, as all the stores open up (some are always open, though) and there will be many sales and discounts. The sellers are in need for shoppers; they even made a 95% discount for what my mom was buying -- you can save a lot, which is great! Cruises arrive every weekend and every Wednesday -- they only stop for a day or two!
Even though you might want to save money, buy water! In a restaurant, buy a glass of water; don’t go for what they give for free and don’t go for tap water, like I did.
The town is safe, but still be careful; at night when you walk on less crowded streets, be careful of thieves. Also, if you rent a car and go downtown, park the car on some smaller street.
Best Way To Get Around:
There is no public transportation and most hotels are away from the town. There are three options for transportation:
You can rent a car, which costs about per day plus insurance (I recommend it). There a few car rentals in town, but rentals at the hotel are best.
Taxis are also another way of getting around; it's about one-way between town and your hotel. The hotel I stayed at (Reef Club Cozumel) was the second farthest from town and it was about a 25-minute ride by car into town. It all depends on the location of your hotel. To the one major attraction, Chankanaab, taxi drivers charge about one-way from any hotel to the town. NOTE: THERE IS NOT MUCH NEGOTIATING ABOUT THE PRICE, BUT TRYING IS NEVER BAD!
The third option is to get around by shuttle, but only few hotels have a shuttle. I think the Intercontinental does and a few more do, but they only go between the hotel, the town, and the airport, and do not stop anywhere near the Chankanaab water attraction/resort.
If you want to go to Cancun from Cozumel (25 minutes by boat), a fare is round-trip, which is cheap compared to a taxi.