Manzalina Beach
As you drive onto the grounds of San Juan, you will arrive at the mansion at some point. There is a guard here who will walk towards you for the entrance fees. You pay 10 NAF per car and then you can visit all three beaches. You then drive on sandy roads to the beaches. There are signs with the names of the beach and which exit to take.
Playa Mansaliña is a beach with coral remnants instead of sand. On the western side there is some sand on the beach. There is also a small cave for shade. It is a special and quiet beach, so definitely worth a visit.