Couple of ideas for you to consider 1. hiring a private shuttle to meet you at the PDC ferry terminal. I frequently have a private shuttle waiting for me at CUN to take me to the PDC ferry, it is a bit more expensive than the bus but is easy & cost effective for larger groups. They may make you a round trip deal or wait for you. Alternatively you can take the local bus back to PDC. 2. In PDC head over to the ADO office and ask when they have a Tulum Direct bus available, it's about 25-30 pesos more than the local bus (you might not get to sit together on the local, since it will have passengers already on board). Take the local back to PDC, since it will be empty.

IMHO the beach on the south side of Tulum is superior to the north.

You can do self-guided tours of the ruins, but get a map/guide.