There are many synonyms for the phrase "not difficult." Words that come to mind include "easy," "simple," "straightforward," and "plain." Other options include "effortless," "uncomplicated," "undemanding," and "facile." Additionally, one could use "manageable," "accessible," "user-friendly," or "approachable" to convey the same sentiment. Depending on the context, phrases such as "no sweat," "a breeze," or "smooth sailing" could also be appropriate. Ultimately, the choice of synonym will depend on the intended tone and level of formality, but there are many options available for conveying the idea of something not being difficult.