I don't know that there are hard-and-fast rules about this, but the tense usage here depends on the style of writing.
If you are writing in a "narrative" style, you would use all past tense. This is the style you would find in a novel or short story, but not usually in a personal letter or in a newspaper article.
Chapter 3: My Exchange Trip
Later that year, I went on an exchange trip to England. I stayed with a nice family, who lived in a big house where they enjoyed playing music. The son, who was called Peter, was an accomplished pianist....
If you are writing about recent experiences in a less literary or more personal style, it is natural to use a mixture of present and past tense.
Thanks for your letter. I just got back from my exchange trip to England. I stayed with a nice family. They live in a big house and they all like playing music. I really hit it off with the son, Peter. He is an excellent pianist, and one night he played through the entire score of The Pirates of Penzance while I sang along....