I assume that you don't need a very formal letter, but I altered the text a bit by grouping into sentences, as the previous format was a bit too artificial. But generally, I tried to stick to the original text as much as possible...
During my trip to Warsaw, I saw impressively tall buildings, about 6 of them, a couple of parks, which I liked because they were large and beautiful, and also a zoo. I liked the way the zoo is made, as there you can see animals without any bars obstructing the view. Monkeys were running and jumping on ropes and trees!
I also saw a couple of hypermarkets [did you mean supermarkets? Hypermarkets are usually found on the outskirts of a town]. They are nothing special; they are practically like the ones in Minsk, only bigger.
A horrible tourist attraction was the in-stadium market [I guess that would be the best choice of words, unless you're referring to a market built actually on a stadium, which I doubt!]. I knew that it exists but I didn't know how awful it looks.
Student or Learner