What to expect
Start with a walk along the beach and malecón. It gives you a better feel for the place than immediately following the first activity sign. For surfing, tell a school your experience and ask where the lesson takes place. Doing nothing is an equally good plan for the afternoon.
How it fits into a journey
Keep this as a free day during your Máncora stay. You do not need a second activity just because there is a blank space in the itinerary.