"在城里寻欢作乐" can be translated into English as "indulge in pleasures and merriment in the city" or "seek fun and enjoyment in the town".
Here are a few more natural-sounding alternatives:
1、 "Have a good time in the city" (a more casual and general expression).
2、 "Enjoy oneself in the urban area" (a slightly more formal phrasing).
3、 "Partake in the city's nightlife and entertainment" (emphasizing the nightlife and entertainment aspects).
The choice of phrase may depend on the specific context and the nuance you want to convey.