One possible version：
Dear Mr and Mrs Johnson,
I’m writing to tell you that I had a safe journey back to Beijing. Thanks to your hospitality, I had a wonderful time in Australia. It was my first time to go abroad and I was naturally a bit nervous when I first arrived. When I saw you at the airport, however, I knew from your warm smile of welcome that everything would be all right. With Jenny’s help, I visited so many places of interest in Australia, which leaves me quite a good memory. I hope some day you will come to visit Beijing and I can return your kindness.
Best wishes,
Li Ming