Hi Learner SC,
I don't think there is an exact translation of "straightforward" the way we use it in English (I could be wrong though). However, we do have expressions that we can use to talk about the ease in which something is to someone.
自然 means "natural," but when used metaphorically, it can mean "easy." For instance: 开飞机对飞行员来说很自然, which means: "Flying a plane, to a pilot, is very easy (natural)."
An idiomatic expression that's used in casual speech is 闭着眼睛, which means "closed eyes" or "with eyes closed" (English has this as well, although it's not always used the same way). For example: 在中国习惯开车的人来到美国后闭着眼睛就能开车了, which translates into: "Someone familiar with driving in China can drive with his/her eyes closed in the United States."
I hope my answer was helpful (even if I didn't answer the question the way you would've liked). Please let me know if you have any more questions =)