Check-in is the moment when a little informed assertiveness pays off more than at almost any other point in a Cancún trip.
Ask about your specific room's beach proximity
Large resorts can be deeper than they look on the map — a "beachfront resort" room might be 5 minutes of walking from the actual sand. Ask which building your room is in and how far it is from the beach, and whether a closer room is available if it matters to you.
Upgrade asks that sometimes work
Arriving at off-peak times (weekday afternoons, shoulder season), mentioning a special occasion genuinely, and asking directly but pleasantly whether any upgrades are available at the current rate are all legitimate approaches. No guarantees, but the ask costs nothing.
Clarify the resort fee situation
Some Cancún resorts add a daily resort fee on top of the quoted nightly rate — if yours does, check-in is when this gets confirmed. If you weren't aware of it, ask what it covers and whether those amenities are things you'll actually use; occasionally they'll waive it for the inconvenience of the surprise.
Confirm current beach conditions
Staff at the front desk will have the most current read on sargassum, water conditions, and which areas of the beach are cleanest right now. Ask directly rather than discovering at 8am the next morning.