The definition of "fun" can be kinda subjective/contextual for individual. In my opinion, I can suggest the most fun cities to travel in each category as below:
ChiangMai - Nature/cultural exp/healing all three-in-one city
Macau - Most fun for casino with affordable budget
Dublin - Most fun drinking city
Shanghai - Most luxurious fun city
Istanbul - Most cultural exotic fun city
Lombok/Gili Island - The best snorkling/diving fun place (not a city...)
Hong Kong - Most fun nightlife city
Seoul, Tokyo - Most fun and safe/clean nightlife city
...
Those selections are just from my personal experiences :)
