Challenge is an interesting and oft-used word, but it's always good to have synonyms in your arsenal. Some related words include difficulty, obstacle, problem, trial, test, contest, competition, confrontation, difficulty, adversity, opposition, and hurdle. Each of these synonyms adds a slightly different spin to the concept of challenge - for example, "obstacle" and "hurdle" suggest a physical or concrete challenge to be overcome, while "competition" and "confrontation" imply an adversarial challenge. Knowing how to swap out synonyms can make your writing more interesting and dynamic, so don't be afraid to experiment with different words.