Cancun international Airport (CUN)
Buses from Cancun run quite regularly. There is also an ADO bus direct from Cancun Airport to Playa del Carmen.
Buses from Playa del Carmen run hourly or so. The Bus station is at the southern end of Fifth Avenue near Playacar. The ADO Bus stops at Xcaret and Xel-ha enroute to Tulum. The Mayab bus stops more frequently enroute to Tulum from Playa del Carmen.
To visit the ruins, get off the bus at the first Tulum stop at the intersection with the access road to the ruins. It’s an easy one mile or so flat walk to the ruins from the intersection.
An alternative to the buses is to catch a “collectivo” van. In Playa del Carmen you can find these on Calle 2 towards Avenida 20. One-way trip costs $50-60 MXN.
It is a very easy drive to Tulum. To get there you take the only highway south from Cancun Airport straight down past Playa del Carmen, Akumal, etc. It’s about a 90 minute drive from the airport to Tulum.
If you drive yourself to the ruins before opening time, it may be a bit confusing as to where to go and what to do. As soon as you park, a man on a bicycle should find you and charge you for parking ($50 MXN). You must go through a sort of half open-air mall (which is empty before 8AM). From there you can either sign up with a tour guide ($20 USD per person?), pay for a shuttle ride to the ruins ($20 MXN), or walk a mile along a road to the ruins. As you approach a stone wall, to the left will be a brown wooden building where you can purchase your ticket into the ruins (57 MXN, an additional 35 pesos if they see that you have a video camera). From there, head along a stone path through the jungle and into the ruins.