Must do Tulum. Rent a car, drive there, beware of cars passing on the highway. To turn left from highway, pull over to shoulder, look both ways, if it is clear, floor it!!
Take the ferry from the Hotel Zone (Playa Tortugas) to Isla Mujeres. A great day trip. (Ferry costs US/adult.) Tours go to the island too and range from basic (US) to all inclusive (up to US-70 per person).
Quick Tips:
Entertainment-plus discount card was accepted at Casa Maguey. Food was fantastic, service was great, mariachi band was cool.
Best Way To Get Around:
We rented a car due to needing a car seat for our 20-month-old. I heard that some rentals have the LATCH system, ours was older and seat installed with seat belts just fine. Walking and public busses are a safe and acceptable alternative.