okasi wo zenbu kami ni tutunde, tomodati he no omiyage ni sita.

Because お菓子 is the direct object of 包んで from 包む (to wrap).

So in:

• お菓子を包む = to wrap the sweets

The sentence starts with the thing being wrapped, so を marks it as the object.

全部 means all or all of it/them.

In this sentence, it most naturally means:

• all the sweets
• or the sweets, all of them

So:

• お菓子を全部紙に包んで
  = wrapping all the sweets in paper

A learner might expect 全部のお菓子 or お菓子全部, and those are possible in Japanese too depending on style and emphasis. Here, 全部 comes after the object marker phrase and sounds natural.

This is a very common question.

With 包む, both に and で can appear, but the nuance is a little different.

• 紙に包む = wrap something in paper
  
  • focuses on what something gets enclosed by
• 紙で包む = wrap something using paper
  
  • focuses a bit more on paper as the material/means

In many everyday situations, both can work, but 紙に包む is very common when talking about the thing that surrounds or encloses the object.

So here:

• お菓子を全部紙に包んで
  = wrapped all the sweets in paper

The て-form is being used to connect actions.

Here the sequence is:

1. wrapped all the sweets in paper
2. made them into a souvenir for a friend

So 包んで links the first action to the second:

• 包んで、友達へのおみやげにした。
  = wrapped them, and made them into a souvenir for a friend

The nuance is usually that the first action happens before the second, or is part of the process leading to it.

友達へのおみやげ means:

• a souvenir for a friend

The structure is:

• 友達へ = to a friend
• の = turns that phrase into something that can modify a noun

So:

• 友達へ
  • の
    • おみやげ
• literally: a souvenir toward/for a friend

This is a very common pattern:

• 日本への旅行 = a trip to Japan
• 母への手紙 = a letter to my mother
• 先生への質問 = a question for the teacher

In English we usually just say for a friend, but Japanese often uses へ + の + noun.

Yes, but the meaning would be less precise.

• 友達へのおみやげ = a souvenir for a friend
• 友達のおみやげ can sound like my friend’s souvenir or a souvenir associated with my friend

So への clearly shows direction/recipient: the souvenir is intended for the friend.

That is why 友達へのおみやげ is the better choice here.

Literally, X にする often means:

• to make something into X
• to decide on X
• to use something as X

So:

• おみやげにした
  = made it into a souvenir = used it as a souvenir/gift

In this sentence, the speaker took the wrapped sweets and treated them as a souvenir to give to a friend.

So it does not necessarily mean they physically transformed the sweets into something new. It means they designated or used them as a souvenir.

Because にする is the fixed grammar pattern here.

• Noun + にする = make something into that noun / choose that noun / use something as that noun

Examples:

• お茶にする = choose tea
• 部屋をきれいにする = make the room clean
• プレゼントにする = use it as a present

So:

• おみやげにした = made it a souvenir / used it as a souvenir

Using just した would not express that same relationship clearly.

Yes, most likely.

Japanese very often leaves out the subject when it is understood from context. This sentence does not explicitly say who did the action.

A natural English translation might add something like:

• I wrapped all the sweets in paper and made them into a souvenir for a friend.

But in Japanese, the subject can be omitted if it is already clear from the conversation.

That omission is completely normal.

Not always exactly.

おみやげ often refers to:

• a souvenir
• a small gift brought back from somewhere
• a present for someone after a trip or visit

In Japanese culture, おみやげ often strongly suggests something you give to other people, not just something you keep for yourself.

So in this sentence, おみやげ is very naturally understood as a gift/souvenir for the friend.

The お is an honorific/polite prefix.

It is often attached to certain common nouns to make them sound more polite or refined. In everyday Japanese, some words are very commonly used with お, including:

• お菓子 = sweets
• おみやげ = souvenir/gift
• お茶 = tea
• お金 = money

Sometimes the version with お is so common that it feels almost like the normal dictionary form in everyday speech.

So here it adds a natural, polite tone.

By itself, the sentence only clearly says that all the sweets were wrapped in paper.

It does not necessarily tell you whether:

• each sweet was wrapped separately, or
• all the sweets were wrapped together in one package

That would depend on context.

If the speaker wanted to be very clear about wrapping them one by one, they might use wording that makes that explicit. As written, the sentence simply states that all the sweets ended up wrapped in paper.