One of the defining features of Tulum is the collection of ancient Mayan ruins that line the coast. For a few pesos, you can enter what used to be a bustling mecca of Mayan culture. Plan on spending at least two hours exploring this one-of-kind marvel. Here's another shot from inside the Tulum ruins:
Less than two hours away (by car) from Tulum is the area of Coba. There you can rent a mountain bike for approximately one US dollar and explore jungle paths even more ancient ruins.
While you're exploring the jungle in Coba, do yourself a favor and climb this pyramid:
It's almost 150' tall! Here's the view from the top: