The phrase "round the bend" typically means to go crazy or insane. A few common antonyms for this expression might include calm, sane, and rational. Other possibilities might include collected, clear-headed, and level-headed. These words all suggest a sense of control and stability, rather than the chaotic and unpredictable behavior of someone who is "round the bend." It's important to note, however, that not all antonyms for "round the bend" would necessarily be positive or desirable. For example, someone who is completely apathetic or emotionless might be considered an antonym for this phrase, but this would not necessarily be a compliment either.