The phrase "warm the cockles of someone's heart" refers to something that brings a deep feeling of happiness and contentment. There are several synonyms that can be used to convey this sentiment, such as "touch someone's soul," "bring joy to someone's spirit," or "fill someone's heart with warmth and love." Other options could include "make someone's day," "brighten their day," or "lift up their spirits." All of these expressions strive to convey the same message: that some action or gesture has had a positive and lasting impact on someone's emotional state, leaving them feeling happy, content, and at peace with the world around them.