Today, I'm going to write about themy days in Denmark when I lived there as an exchange student. Today, I'm going to write about my days in Denmark when I lived there as an exchange student.

Adding “my days” makes the sentence more natural and personal.

Today, I'm going to write about themy days in Denmark when I lived there as an exchange student. Today, I'm going to write about my days in Denmark when I lived there as an exchange student.

Today, I'm going to write about the daystime in Denmark when I lived there as an exchange student. Today, I'm going to write about the time in Denmark when I lived there as an exchange student.

"days" isn't wrong, but it gives an impression of a far off time. Like a very elderly person looking back at their childhood.

Today, I'm going to write about themy days in Denmark, when I lived there as an exchange student. Today, I'm going to write about my days in Denmark, when I lived there as an exchange student.

It was one of the best periods inof my life. It was one of the best periods of my life.

Use “of my life” instead of “in my life.”

It was one of the best periodtimes in my life. It was one of the best times in my life.

Times sounds better here.

Luckily, I was able to spend a summer vacation there, soand I visited many places inaround the country. Luckily, I was able to spend a summer vacation there, and I visited many places around the country.

And” flows better than “so.” “Around the country” is more natural than “in the country.”

Luckily, I was able to spend a summer vacation there, so I visited many places in the country. Luckily, I was able to spend a summer vacation there, so I visited many places in the country.

or: "...within the country"

While I wandered through the cities, I realized that time flowedseemed to flow more slowly than in Tokyo. While I wandered through the cities, I realized that time seemed to flow more slowly than in Tokyo.

Add “the” before “cities.” Use “seemed to flow more slowly” for smoother, native-like phrasing.

While I wandered through cities, I realized that time flowed more slowly than in Tokyo. While I wandered through cities, I realized that time flowed more slowly than in Tokyo.

While I wandered through cities, I realized that time flowed more slowly than in Tokyo. While I wandered through cities, I realized that time flowed more slowly than in Tokyo.

While I wandered through cities, I realized that time flowed more slowly than in Tokyo. While I wandered through cities, I realized that time flowed more slowly than in Tokyo.

While I wandered through cities, I realized that time flowed more slowly than in Tokyo. While I wandered through cities, I realized that time flowed more slowly than in Tokyo.

Nice use of "slowly" I always like when I see people use adverbs correctly, as many natives can't. Good job!

TheyPeople normally leave work in the evening and spendenjoy their ownfree time, for instance, drinking coffee atby the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city. People normally leave work in the evening and enjoy their free time, for instance, drinking coffee by the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city.

“People usually” is more natural than “They normally.” “Enjoy their free time” is smoother than “spend their own time.” “By the riverside” and “taking walks” are more usual.

They normally leave work in the evening and spend their own time, for instance, drinking coffee at (or by) the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city. They normally leave work in the evening and spend their own time, for instance, drinking coffee at (or by) the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city.

They normally leave work in the evening and spend their own time, for instance, drinking coffee at the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city. They normally leave work in the evening and spend their own time drinking coffee at the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city.

"for instance" here comes across as a bit "filler-y" and not required

They normally leave work in the evening and spend their own time, fhave time to themselves. For instance, drinking coffee at the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city. They normally leave work in the evening and have time to themselves. For instance, drinking coffee at the riverside, talking with their friends at pubs, or taking a walk in the huge parks in the city.

I really envy them, whileeven though I know that not all people can do that. I really envy them, even though I know that not all people can do that.

“Even though” is more natural than “while.”

I really envy them, whilealthough I know that not all people can do that. I really envy them, although I know that not all people can do that.

"Although" is more natural for this kind of conjunction where the second part of the sentence softens/lightly contradicts the first.

I really envy them, whilefor I know that not all people can do that. I really envy them, for I know that not all people can do that.

*If you have time and come up with more "native-like expressions," please teach me, thanks.* *If you have time and come up with more "native-like expressions," please teach me, thanks.*

*If you have time and come up with more "native-like expressions," please teach me, thanks.* *If you have time and come up with more "native-like expressions," please teach me, thanks.*

*If you have time and come up with more "native-like expressions," please teach me, thanks.* *If you have time and come up with more "native-like expressions," please teach me, thanks.*