>I spent my at holidays in Gdańsk.
Bez "at". Poza tym chyba jest jakaś różnica między holiday a holidays.
>The sun was shining all days
Napisałabym: every day.
>I was often swimming, playing volleyball, reading books and watching TV.
Nie może być "often" z czasem continuous.
>I met many new friens there.
I met there a lot of new friends.
>I sent also
I also sent them a lot of posstcards.
>which I never forget.
which I will never forget.