Sure! Here’s how you can complete the postcards with a/an, the, or zero article (X):

Hi Nga,
You know what? I’ve done it, at last! I’m on (1) **a** Explore Europe holiday to see ten different countries in (2) **X** Europe. I watch them go past from (3) **the** window of (4) **a** luxury air-conditioned coach and stay at top campsites on (5) **X** way. So far I’ve visited (6) **the** Eiffel Tower in Paris, (7) **the** historic Sistine Chapel in (8) **the** Vatican, taken (9) **a** gondola ride through (10) **the** canals in Venice, eaten (11) **an** apple strudel in Vienna and listened to (12) **the** Krizik Singing Fountain in Prague. But there’s lots more on our itinerary. At (13) **the** end of this week we will have (14) **a** lot of fun in Tatralandia Aquapark in Slovakia.
Bye for now, Minh.
P.S. You should also go on (15) **a** touring holiday in the future.