Osaka City in Japan

In general titles are capitalized with exception of grammatical articles.  You can get away with capitalizing everything three characters or longer.

Food in this context is an uncountable noun, so it cannot be pluralized.  I can eat "two meals" or "too much food", but cannot "eat two foods".

Central is an adjective, therefore Shinsaibashi can be central to Osaka, or Shinsaibashi can be known as the central district of Osaka.

I recommend that you might wanna go to Universal Studios Japan. I think it’s the best amusement park in Japan. Harry Potter area is the most popular area there. Its castle is very realistic and the roller coaster and riding machine are very exciting. I like the Jurassic Park area too. The roller coaster and riding the boat are very interesting. Osaka is famous for its food, such as Takoyaki and Okonomiyaki. They are so delicious.

Saying "I recommend that you might want to..." is redundant.  Say either "I recommend that..." or "You might want to..."

I recommend "Harry Potter is the most popular area there."

I recommend "Riding the roller coaster and riding the boat are very interesting." or "It is fun to ride the roller coaster and boat."  It is awkward to mix a noun and an action the way you've done.

You can go to other famous places easily from Osaka, which are Kobe, Nara and Kyoto. There are many traditional Japanese shrines and temples in Kyoto. If you go to Nara, you can see Big Japanese Buddha which is called Daibutsu.

"Which are" implies an complete list, but Japan has more than three other famous places.  Use "Such as Kobe, Nara, and Kyoto" or "Including Kobe, Nara, and Kyoto" instead.

Big must be capitalized because it is part of the name.

I'd also recommend that you break your writing into paragraphs, as has been done in this correction.

Osaka city in Japan