In 我去北京旅行, is 北京 the object? If not, what *is* it? I am a native speaker who is learning about how Chinese grammar works. I know that a simple sentence contains a subject (主语), a verb (谓语?) and an object (宾语).


  我(subject)吃(verb)饭(object)。


I am also aware that there are other "X语" like 状语, 表语, 补语 etc. I have little to no understanding of how they come into play.

I came across this sentence:


  我去北京旅行。


Obviously, 我 is the subject, 去 is the verb. But which is the object? 北京 or 旅行? If either of them is the object, then what is the other one? 补语?
This is an example of a serial verb construction. Mandarin Chinese can have a transitive verb-object followed by an intransitive verb. The object of the transitive verb can become the subject of the intransitive verb. Or, in this case, the intransitive verb can share the subject with the transitive verb. (Note: In English, 'go' is an intransitive verb. In Mandarin, it's transitive.) 

我去北京 is clearly a subject-verb-object. 旅行 is an intransitive verb. As such, it can be a 'serial' (i.e. added in a series) after a transitive verb and its complement.