Here's the completed conversation with the correct Past Simple forms:
Nick: What did you do (you/do) in the summer?
Eric: 1. I took (take) a trip around Europe by train.
Nick: (2) Did you go (you/go) on your own, or with some friends?
Eric: A couple of friends (3) came (come) with me.
Nick: How many countries (4) did you visit (you/visit)?
Eric: I (5) went (go) to six or seven countries. I (6) had (have) a great time, and I really (7) loved (love) all of them.
Nick: Which one (8) did you like (you/like) most?
Eric: Sweden, I think. I (9) enjoyed (enjoy) exploring the marvellous countryside and I (10) took (take) lots of photographs.
Nick: When (11) did you arrive (you/arrive) back home?
Eric: Last week, I'm still rather tired.