Laguna Kaan Luum is a bright, shallow lagoon south of Tulum, known for the dark circular cenote that drops away in its centre. The colour contrast is the hook: pale turquoise water around the edges, deep blue in the middle.
It is a good low-key alternative when Tulum's beach scene feels too crowded or expensive. Go early, follow swimming restrictions around the cenote, and bring what you need since facilities are simpler than at the big coastal clubs.