In Puerto Rico, it is traditional for children to fill a box with fresh grass or hay and put it underneath their bed, for the Wise Men's camels. The three kings will then take the grass to feed the camels and will leave gifts under the bed as a reward (Traditionally, you'll find carvings and artwork of the three kings on horses instead of camels; that's because the country folk in years past didn't know what a camel was). These traditions are analogous to the customs of children leaving mince pies and sherry out for Father Christmas in Western Europe or leaving milk and cookies for Santa Claus in the United States.
Today, children typically get their main presents on the 25th, but there's always a smaller, humbler, and perhaps even more rewarding gift reserved for Three Kings Day. Many people on the island see the Three Kings as a more pure representation of the faith and season than Santa Claus. The topic of Santa versus The Three Three Kings has been politicized. Think of it as the Puerto Rico's war on Christmas. Here El Gran Combo De Puerto Rico sing La Protesta De Los Reyes where The Three Kings make their protest.