If they started by looking at Cancun, they might be interested in getting one (or more) of the houses at Playa del Secreto located a little ways south of Cancun airport and just north of Playa del Carmen. It is a secluded and tranquil spot with on site services provided. Been there twice - we first did a house share with our kids and another family of 4 -- and we didn't use (rent) the entire villa. The second time we did rent the entire place including the 3rd floor and we had 3 families of 4. Stayed in Villa Sarita both times (it faces the beach, has its own pool, outdoor bar area and is just steps to the sea) I will say it's been some time since we've been there but it was pretty cool. http://www.playasecreto.com/