好きな海外旅行

私の一番好きな海外旅行は初めての海外旅行もだった。高校生の時にヒューストンでNASAの「ユナイテッド・スペース・スクール」に行った。目的は火星へのミッションを設計することだった。たくさん友達を作って楽しかった。

My favourite holiday was also my first holiday. When I was a high school student, I went to NASA's "United Space School" in Houston. I made lots of friends and had a great time.

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I'm not sure on which word to use for "holiday/vacation". Most words for "vacation" seem to suggest time off work more than just a trip somewhere to me. Which word would you use to say you're going on a holiday to some other country, say, but not necessarily because you have time off work?
I also am not really sure how I would say "my first X" or "my only X". What are some different words/phrases you would use for this and how do they differ?

ありがとうございます。