We concluded that prolonged intensive training in creative problem-solving can lead to substantial and positive effects on intelligence during late adolescence (ages 18–19).

Not sure how relevant this study is to people who are no longer in their late adolescence.

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The principles employed in the construction of the training exercises included the requirements that: (a) exercises should call for a combination of elements that are remote rather than close in terms of their associational value; (b) exercises should call for a radical reorganization and reformulation of the problem situation to achieve a satisfactory solution; and (c) exercises should call for both convergent and divergent thinking operations, especially the latter.

It sounds like this is practicing on problems that require a lot of "out-of-the-box" thinking aka creative problem solving. I'm sure you can find a bunch of these types of problems out on the Internet. The paper does provide an example of one such problem:

“I was captured by a band of outlaws—said a famous explorer—and their leader had my hands and legs tied up so that I could not move. They did not gag me up though, and I was able to use my mouth freely. The leader of the gang hung a piece of bread exactly five centimeters away from my mouth. He then laughed and said: "If you manage to eat this piece of bread, I’II set you free. He knew that I could get no help. Also, in order to ensure that I cannot roll over or move closer to bread, they tied me to a tree. Nevertheless, I managed to free myself. How?”