"在城里寻欢作乐" can be translated into English as "indulge in pleasures and merriment in the city" or more colloquially, "have a good time and party in the city".
The first phrase emphasizes the idea of indulging in various forms of enjoyment and festivity, while the second is more casual and conveys the sense of having fun and socializing. Choose the one that best fits the context and tone you wish to convey.