This is a basic option in a great location (close to the beach but not too close), but it's definitely a get-what-you-pay-for experience that could benefit from upgrades.

Carlos needs to buy a new mattress, a new sofa bed, cereal bowls, some kitchen supplies (pepper?), a better internet connection, and some cheap beach toys.

He should start a guestbook and a provide map with local recommendations/info on the area (even just one printout).

Carlos should also be on time for the arrival of his guests or, at the very least, make sure one of his phones are on.