Like Cabo Lucas, Cancun has the usual overpriced watersports activities, so we opted to have a chilled morning for once and went onto the resorts along zona hotela, using their facilities for free (no-one ever asked us if we were staying there). The beaches are nicer further north you go, but Playa Delfines was quiet, so we stayed around that area.
In the afternoon we went diving with Scuba Cancun, to see the underwater museum. There was a small house, a car, sculptures, frames,etc. We saw a lot of fish, a turtle and sea spiders. For $48USD pp, it was worth it.
Later, we went to Playa del Carmen, made camp, had a birthday meal, followed by a cake with candles.