What's the best way to improve Chinese after mastering the fundamentals?
Thanks for everyone's comments. I think I got some answer to this question.

To improve Chinese beyond intermediate level, one can "use the language in context". 

Single-person activities include


Reading Chinese in cartoon book with a topic you are interested in or familiar with
Watch Chinese drama/movies or translated drama/movies with Chinese voice/subtitles
Play Chinese RPG games in which you would see how the language is used in practice, and how people with different social status address each other.
Switch your phone or computer to Chinese language, and you can learn some new words and sentences. (though I think this one is intermediate level).


Multi-person activities:


Talk and interact with other learners or native speakers.