suru verb (special), transitive verb
give free rein to (imagination); let one's imagination run wild
Used specifically with 想像力 (imagination) to mean allowing it to work freely and vividly. Often appears in the phrase 想像力を逞しくする.
子供の頃は、よく想像力を逞しくして遊んだものだ。
When I was a child, I used to play while giving free rein to my imagination.
彼は想像力を逞しくして物語を作った。
He let his imagination run wild and created a story.