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

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!

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.

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

Today, I'm going to write about the daystime 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.

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

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.

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

I really envy them, whilealthough 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.

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

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

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

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.

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

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

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

It was one of the best periods inof my life.

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

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

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

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

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

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 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.

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

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

It was one of the best periodtimes in my life.

Times sounds better here.

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

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