The phrase "thinking back" can be interchangeably replaced with several synonyms. One alternative is "reminiscing," which refers to the act of reflecting on past experiences. Another word that could be used is "recalling," which means to call to mind a past event or memory. Alternatively, "retrospecting" could also be used, which can be defined as reflecting on or reviewing the past. One more possibility is "remembering," which means to bring to mind something from the past. Overall, these synonyms help to convey the idea of looking back on past experiences, whether for reflection, memory, or analysis.