Five days will give you enough time to explore many of the best parts of the Caribbean Coast. If you use Playa del Carmen as a base, you can try the following itinerary.
Start with a day-long visit to the Xcaret eco-park. Spend the morning of Day 2 at the gorgeous cliff-side ruins of Tulum; in the afternoon cool off at Xel-Há or at one of the numerous cenotes along Carretera 307. On Day 3, explore the beaches at Paamul and Xpu-há and continue on to Akumal and Yalkú. Spend Day 4 at the Maya village of Pac Chen in the morning and, after lunch, head for the ruins of Cobá. On Day 5, take a tour of the Reserva de la Biosfera Sian Ka'an.
Visit the Travel Talk forums for help on planning your trip