[ 你好  Your English looks excellent - way, WAY better than my beginning 普通話, LOL!

One general note to start with.  In English, we ALWAYS put at least one blank "Space" after the end of each sentence (after the ".", "!" or "?")  I say "at least" one space because some of us older folks put in two spaces.  We do not have a clever Chinese 。 to put the space in for us!  Compare:  >|.|< & >|。|<

MY KEY: red - omit,  green - insert/replace, [blue - suggeseted improvement], [grey - my comments]  

Happy May second,

We are having a holiday on account of the International Labour Day.  I visited the China Ethnic Museum yesterday.  I felt happy as soon as I arrived here there, for there it was quiet [ because it was quiet there "for" is correct but sounds very formal, we usually use "because" ( or "'cause" in slang speech) ] and very few people [ were there ].  My friend and I entered the museum starting and started our sightseeing.  We saw the Tibetan, the Mongol and many other Ethnic Minorities Minority displays.  Different nationalities have their own and special architecture, dress, and diet, which is very interesing. I was astonished at their cultures.  I appreciated all the scenes I saw.  When I inadvertently glanced at the Bird's Nest Olympic Stadium, I was in a high spirits.  [ I'm not quite sure what to say here. "Inadvertently" means "without intention; accidentally" - a bit of a peculiar thought in this circumstance for a 'western mind' (different cultures think differently**).  Also, we only commonly ocasionally tend to attribute "high spirits" in the third person ("She was in high spirits.")  I have no idea why! This is no rule that I know of, just common usage.  As soon as I read this correct sentence, I think: 'Not a native English speaker.'  8^)  I hate to steer you away from a fine word like "inadvertently", but it's not a common*** English word and, it doesn't quite fit here.  A more culturally typical sentece might go like this: When the Bird's Nest Olympic Stadium first caught my eye, I was thrilled! "caught my eye" is a common English idiom - you may know it]  We spent the whole afternoon staying [ "staying" is correct but not needed or used (it is implied) ] in the museum. Definately, it's worthy. [ We would not say this; we might say it's worth your time. or it's worth a visit. Now, I'm over 60 and it IS possible that "it's worthy" IS a current idiom amongst my kids (in their late 20's).  I will message my daughter & ask her. ]

** I'm just visiting my 弟弟 for a week and he always says "Different" is NOT a synonym fpr "Wrong!" - and he is RIGHT!  Two cultures may do something differently, but that does NOT mean that one of the cultures is wrong.

*** However, "inadvertently" is one of my favorite words.  I can say I stupidly stubbed my toe.  or, I can say I stubbed my toe inadvertently.  This big word makes my stupid deed SOUND intelligent!!  LOL! ]