ryokou de katta okasi wo tomodati he no omiyage ni simasu.

It breaks into these parts:

• 旅行で = on the trip / during the trip
• 買ったお菓子 = the sweets I bought
• を = marks that noun as the thing being used/chosen
• 友達へのおみやげ = a souvenir for a friend
• にします = will make it into / will use it as / will choose it as

So the sentence structure is basically:

[the sweets bought on the trip] を [a souvenir for a friend] にします

That pattern means:

I’ll use the sweets I bought on the trip as a souvenir for my friend.

Here, で marks the place or setting where the action 買った happened.

So:

• 旅行で買った = bought on the trip / bought during the trip

Even though 旅行 is not literally a physical location like 店 or 日本, Japanese often uses で with events or situations to mean in/at/during that context.

So 旅行で here is not for a trip. It means while on the trip.

Japanese uses a relative clause directly before the noun it modifies.

So:

• 買ったお菓子 = sweets that I bought

There is no need for a word like that, which, or who.

A very literal way to see it is:

• 買った = bought
• お菓子 = sweets

Put together, Japanese naturally gets:

• bought sweets = the sweets I bought

This is one of the most common sentence patterns in Japanese.

Because the sentence talks about two different time frames:

• 買った = the buying already happened in the past
• します = the current or future decision/action

So the meaning is something like:

• I bought these sweets on the trip, and now I’ll use them as a souvenir for my friend.

Japanese often mixes tenses this way when one action happened earlier and another is happening now or later.

This is because of the grammar pattern:

A を B にする

This means:

• make A into B
• choose A as B
• use A as B

In this sentence:

• A = 旅行で買ったお菓子
• B = 友達へのおみやげ

So:

• 旅行で買ったお菓子を友達へのおみやげにします
• I’ll make/use the sweets I bought on the trip into a souvenir for my friend

The を marks the thing being designated or chosen.

Here, にします does not mean physically transforming sweets into a souvenir in a dramatic way. It means:

• decide on
• choose as
• use as

So the nuance is:

• I’ll make these sweets my souvenir for my friend
• I’ll use these sweets as the souvenir for my friend
• I’ll choose these sweets as the souvenir for my friend

This is a very common use of N にする in Japanese.

For example:

• 飲み物はお茶にします。
  I’ll have tea.

Same idea: choosing something as the option.

友達への is made of:

• 友達 = friend
• へ = toward / to
• の = links the whole phrase to the next noun

So:

• 友達へのおみやげ = a souvenir for a friend

This pattern, N への N, is common when the second noun is directed toward the first noun:

• 母への手紙 = a letter to my mother
• 先生への質問 = a question for the teacher
• 友達へのプレゼント = a present for a friend

So here, おみやげ is directed to the friend.

Because the phrase is modifying a noun: おみやげ.

Japanese often uses patterns like:

• への
• での
• からの
• との

to connect a marked phrase to a following noun.

So:

• 友達へのおみやげ = a souvenir for a friend

You cannot say:

• 友達にのおみやげ ✘

That is not grammatical.

If you wanted to avoid への, you could say something like:

• 友達にあげるおみやげ = a souvenir I’ll give to a friend

But in this sentence, 友達へのおみやげ is the neat noun phrase.

Because 友達のおみやげ can be ambiguous.

It might suggest things like:

• a souvenir belonging to the friend
• a souvenir from the friend
• the friend’s souvenir

But 友達へのおみやげ clearly means:

• a souvenir intended for the friend

So への makes the recipient relationship much clearer.

They are similar in meaning here, because both can point toward a destination or recipient. But with noun-modifying phrases like this, への is the normal pattern.

Also, へ often gives a slight sense of direction toward someone, which sounds natural with things like:

• gifts
• letters
• messages
• souvenirs

So 友達へのおみやげ is a very natural expression.

The subject is omitted, which is very common in Japanese.

Depending on context, it is probably:

• 私は = I
• or possibly 僕は / 俺は / 私たちは, etc.

Japanese leaves out subjects whenever they are already understood.

So even though English usually wants I, Japanese often does not need it.

The お is an honorific/polite prefix, but in many everyday words it becomes part of the normal word people commonly say.

So:

• おみやげ
• お茶
• お金
• お菓子

are all very common.

Saying みやげ is possible in casual speech, but おみやげ is more standard and natural in many situations.

You may also see it written as:

• お土産

The two sentences focus on different things.

• 友達におみやげをあげます
  = I’ll give my friend a souvenir.
  Focus: the act of giving.

• 旅行で買ったお菓子を友達へのおみやげにします
  = I’ll use the sweets I bought on the trip as a souvenir for my friend.
  Focus: deciding what the souvenir will be.

So this sentence is especially natural when you are choosing what to give.

Yes, that is the literal grammar, but it sounds unnatural in English.

Japanese N を N にする often uses make ... into ... literally, but in many real sentences the natural English is:

• use ... as ...
• choose ... as ...
• have ... as ...
• decide on ... as ...

So here, a natural translation is:

• I’ll use the sweets I bought on the trip as a souvenir for my friend.

That captures the Japanese meaning better than a word-for-word translation.