In Cancun, use the city bus--65 pesos per trip, no matter how long. Tell the driver where you want to go in a short phrase, i.e. "Sr. Frog's," then hang on and sit down.
Outside of Cancun (Riviera Maya/Playa), get a rental car. I went online the week we were in Cancun and rented through Costco and picked up at the Hilton. It was approximately /day for a four-door stick shift with A/C. The hotel wanted /day. We had much more fun eating and roaming in Playa than being stuck at the hotel. Cab fare to Playa was one-way, and we were glad we did not have to stand on the side of the highway in the dark to wait for the bus. The ride from the Mayan Palce was 15-20 minutes. It also allowed us to explore the water parks and Tulum.
Have a fun day at Xel-Ha (shell ha), snorkel the lagoon, or spend all day in the water if you want. There are cenotes, a rope swing, a diving cliff, and swimming with the dolphins. If you plan on staying all day, get the all-inclusive for food and drinks (including beer, drinks, and ice cream), towels, lockers, and snorkeling equipment. There are five or six restaurants and plenty of gift shops. You need biodegradable sun tan lotion--they hold yours and give you a small pack in exchange (you get yours back when you leave).
X-Caret is another fun park. Float the rivers, swim the lagoon, snorkel, see the animals, visit the aquarium, and swim with the dolphins. There are a lot of extras when you're in the park, and you have to rent snorkel gear, etc. We brought snorkeling equipment and towels. I didn't see any all-inclusive here. STAY FOR THE NIGHTTIME SHOW, done very well and professionally. We paid each for dinner, including small appetizer, salad, main dish, and dessert, plus two drinks. The show lasts till 9ish. We thought it was worth it.
Explore some beaches near Tulum, go to Cozumel for the day, and include a snorkeling tour there, for 3 hours.
Quick Tips:
Groceries and supplies in Cancun - Wal-Mart, Sam's Club, Costco, Chederaux. Take the bus in and take a cab home (cab to our condo was ).
Don't let the solicitations bother you. Just say "No, thanks." Ask for the check at the restaurant, and they'll let you hang out forever.
Take small bills--lots of , , etc. If you pay with a large bill (dollars), they like to give you change in pesos. We exchanged fairly small amounts of money at the condo, and their exchange rate was good.
Book your transportation from the airport to the condo/hotel prior to leaving. I paid /person. The airport is a complete zoo! From the hotel to the airport, a cab costs . You really do need the two hours before your flight just to find the end of the line for your airline.
Have fun!
Best Way To Get Around:
Bus, bus, bus. They will give you change. 65 pesos/person (.65). Get some coins from the hotel/condo. Get on, pay, tell them where you want to go (short phrase, not too much talkie, talkie), and HANG ON! The buses are reasonably clean and not scary.
Outside of Cancun, I would get a car. We weren't sure until we got there, so I just booked online while we were there and picked up at a different hotel.