祐希さんの回答に加えて：
What do you enjoy doing most? は「何をするのが一番楽しいですか？」になります。
enjoy は「[楽しむ](https://eikaiwa.dmm.com/uknow/questions/33272/)」という意味ですね。
例：
May: What do you enjoy doing most?
何をするのが一番楽しいですか？
Erik: I enjoy playing tennis.
テニス[をするのが好きですね](https://eikaiwa.dmm.com/uknow/questions/135239/)。
What's your favorite way of spending your free time?
When do you feel the most joy?
When are you happiest?
（訳：一番[喜びを感じる](https://eikaiwa.dmm.com/uknow/questions/113390/)ときはどんな時？）
What's your favorite way of spending your free time?
(訳：暇な時、何をするのが一番[好き](https://eikaiwa.dmm.com/uknow/questions/135239/)？）
答えるときは、
I feel the most joy when ~~、
I'm happiest when ~~
My favorite way of spending my free time is ~
と答えます。
【例】
I feel the most joy when gathering with my family.
家族で集まる時に最も喜びを感じます。
I'm happiest when I travel．（happiest は happyの最上級です。）
旅行をしている時が最も幸せです。
My favorite way of spending my free time is listening to my favorite music.
暇な時は好きな音楽を聴くのが好きです。
If you want to ask someone wen they feel the most joy, you can ask that question several ways, such as the three suggestions above. However, you can also ask:-
1. When are you the happiest?
2. What is your favorite pastime?
3. What are you into?
嬉しいとどんな時に感じのかを尋ねたい場合は、上記の3つの言い方で質問をすることができます。
しかし、以下の言い方でも尋ねる事ができます。
1.When are you the happiest?
（どんな時あなたは幸せですか？）
2.What is your favorite pastime?
（あなたのお気に入りの趣味は何ですか？）
3.What are you into?
（あなたは何にはまっていますか？）
"Happiest" is a subjective thing and means something different for everyone, depending on your life experience and your attitude to everyday things is positive or negative.
To be content = mentally or emotionally satisfied with things as they are.
"Maria seems to have adapted to married life very well!"
"Yes, I think she has adapted very well to her new role as a queen."
"Happiest" とは主観的でみんなその意味するものは違う形容詞だ。何故ならみんな人生の経験は違うし、何が良いか悪いかなんて人によるからです。
To be content = 精神的にも精神的にも今の物事に満足している状態
例："Maria seems to have adapted to married life very well!" マリアは結婚生活にうまく順応したようだ。
例"Yes, I think she has adapted very well to her new role as a queen." うん、女王としての薬をしっかりこなしているね
This asks another person about the emotional joy that they have in their life, and what/who/how they achieve it.
'When are you happiest?' asks people to think about their happy times in their lives and share it.
'What do you enjoy doing?' asks people to think about hobbies, activities or events in their lives that they enjoy/like.
'What brings your happiness?' asks a broad question so that people can consider people, places, activities, things that makes them happy.
生活の中での精神的な喜びと、それが何・誰によってどんな風に得られるかを相手に尋ねる表現です。
'When are you happiest?'
これは、生活の中での幸せな時間について考えてもらい、それを教えてもらうための表現です。
'What do you enjoy doing?'
生活の中で楽しんでいるあるいは好きな趣味、活動、出来事などについて考えてもらう表現です。
'What brings your happiness?'
人、場所、活動など、その人を幸せにするものが何かを考えてもらう幅広い質問です。
*When are you your happiest?
This is the moment you feel very happy.
For example:
A:When are you your happiest?
B:When my husband comes from work.
*What brings you the most joy?
This means what can happen that can make you happy.
For example:
A:What brings you the most joy?
B:When my son tells me that he loves me.
*When are you your happiest?
これはあなたが非常に幸せだと感じる瞬間です。
【例文】
A:When are you your happiest?（いつが一番幸せですか？）
B:When my husband comes from work.（夫が仕事から帰った時です）
*What brings you the most joy?
これはあなたを幸せにするものを意味します。
【例文】
A:What brings you the most joy?（あなたを一番幸せにするものは何ですか？）
B:When my son tells me that he loves me.（息子が私を大好きだと言う時です）
All three of these sentences, more or less, have the same meaning when talking about what someone likes to do or enjoys the most. The main grammatical feature to notice in all three of these sentences is that we are using the superlative form with -est ending on adjectives or having the word, "most," come before the adjective.
To ask someone when they feel the most joy, you can say:
"When do you feel the most joy?"
"When are you happiest?"
"When do you feel elated?"
「何をしている時が一番楽しいですか」は、次のように言えます。
"When do you feel the most joy?"（何をしている時が一番楽しいですか）
"When are you happiest?"（何をしている時が一番楽しいですか）
"When do you feel elated?"（何をしている時が楽しいですか）
When you want to ask someone when they feel the most joy; then you may say it in the following ways:
-When are you at your happiest?
-When do you feel the most joy?
これらのフレーズは、人に何をしているときが一番楽しいかを聞く表現です。
-When are you at your happiest?
（どんな時が一番幸せですか？）
-When do you feel the most joy?
（一番楽しいと感じるのはどんな時ですか？）
If you wanted to ask someone when they feel the most joy, you could ask them any of the following questions for a direct answer: "When are you your happiest?", "What makes you feel the most joy and why?" or "What makes you happiest in life?".
「何をしている時が一番楽しいですか」は次のように言えます。
"When are you your happiest?"（何をしている時が一番楽しいですか）
"What makes you feel the most joy and why?"（何をしている時が一番楽しいですか。そしてそれはなぜですか）
"What makes you happiest in life?"（何をしている時が一番楽しいですか）
"When" is the best question word for this particular question even though the person isn't going to describe a time but rather an experience. You want to use a superlative (adjective 'with -est ending) to describe the best or maximum quality. I hope that this helps! :)
If you want to ask someone what brings them the most joy/happiness in their life, you can ask them by saying something like "What makes you the happiest in life?" or "What brings you the most joy in life?" these are both polite and easy ways to ask someone what makes them happy.
「何をしている時が一番楽しいですか」は次のように言えます。
"What makes you the happiest in life?"
"What brings you the most joy in life?"
（何をしている時が一番楽しいですか）
どちらもシンプルで丁寧な言い方です。
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