"Lean back" can be replaced with several synonyms, depending on the context or situation. For a casual and relaxed environment, one can use "recline" or "lounge," which both imply a laid-back manner. "Slouch" is another alternative that might denote a relaxed or lazy attitude. On the other hand, "tilt back" or "inclining backward" could be used in more formal settings or when referring to a physical action. "Retreat" could be a poetic and metaphoric substitute, implying a retreat into oneself or isolation. Lastly, "step back" might connote a mental or emotional distancing from a situation or problem. Replacing the word "lean back" with one of these synonyms can add depth and meaning to your text and enrich your vocabulary.