Learn Hardcore Japanese: renkyuu ni ha tomodati to issyo ni kaigairyokou wo sitai desu. - During a long holiday, I want to travel abroad with a friend.

Marks the time phrase as the topic of the sentence:
- 連休には、友達と一緒に海外旅行をしたいです。
  → As for the long holiday, I want to travel abroad with my friends.
Often adds a slight contrast or emphasis:
- Implied: Maybe not at other times, but *during the long holiday, I want to travel abroad.*

You could say 連休に友達と一緒に海外旅行をしたいです, and it would still be correct and natural.
連休には just highlights that period a bit more, like saying “(Especially) during the holidays…”

Why is there a に after 一緒 (一緒に)?

一緒に is a fixed phrase meaning “together (with someone) to do something.”

The pattern is:

A と一緒に B をする
→ do B together with A

So in this sentence:

友達と = with friends
一緒に = together (adverb modifying the verb)
海外旅行をしたいです = want to travel abroad

All together:
友達と一緒に海外旅行をしたいです。
→ I want to travel abroad together with my friends.

You can’t drop the に here; 一緒に is treated like one word.

Isn’t 友達と and 一緒に saying the same thing twice? Why both?

They overlap in meaning, but they’re not exactly the same:

友達と = with my friends (marks who you’re with)
一緒に = together (emphasizes doing the action jointly)

Combinations:

友達と海外旅行をしたいです。
→ I want to travel abroad with my friends.
(Natural; “with my friends” is clear.)
一緒に海外旅行をしたいです。
→ I want to travel abroad together.
(Together with someone, but not clear who unless context tells you.)
友達と一緒に海外旅行をしたいです。
→ I want to travel abroad together with my friends.
(Clear and slightly more emphatic.)

So using both is very normal and sounds natural, especially in a stand‑alone sentence.

What exactly is 海外旅行? Is it a verb or a noun? Why does it take を?

海外旅行 (かいがいりょこう) is a noun meaning “overseas trip / traveling abroad.”
It’s made from:

海外 = overseas
旅行 = trip / travel

Japanese often makes a verb by combining a noun with する:

勉強 + する → 勉強する (to study)
仕事 + する → 仕事する (to work)
旅行 + する → 旅行する (to travel, to take a trip)
海外旅行 + する → 海外旅行する (to travel abroad)

Since 海外旅行 is the object of する, you mark it with を:

海外旅行をする = (to) travel abroad
海外旅行をしたい = want to travel abroad

Why is it したいです and not just したい or します?

する = to do
したい = want to do (the –たい form expresses desire)
したいです = polite version of したい

Nuance:

海外旅行をしたい。
→ Casual “I want to travel abroad.” (to friends, family, etc.)
海外旅行をしたいです。
→ Polite “I want to travel abroad.” (safer with teachers, coworkers, etc.)
海外旅行をします。
→ “I’ll travel abroad / I travel abroad.”
This states the action, not the desire, so it changes the meaning.

In this sentence, we are talking about what the speaker wants, so したいです is the correct choice.

Why is です used after したい? Isn’t です usually after nouns and な‑adjectives?

The –たい form (したい, 行きたい, 食べたい, etc.) behaves like an い‑adjective:

高いです = it is expensive
楽しいです = it is fun
したいです = I want to do (it)

So adding です to したい makes it polite, just like with other adjectives:

Plain: したい。
Polite: したいです。

Grammatically, you can think:
“I am in a want‑to‑do state” → したいです

Who is the subject here? There is no 私 in the sentence. How do we know it means “I want to…”?

Japanese often omits the subject when it’s obvious from context.

In this sentence:

連休には友達と一緒に海外旅行をしたいです。

By default, when someone says ～したいです about desire, it’s understood as the speaker’s own desire → “I want to…”

So the full meaning is:

(私は) 連休には友達と一緒に海外旅行をしたいです。
→ I want to travel abroad with my friends during the long holiday.

Adding 私は is grammatically fine but often unnecessary unless you need to emphasize I (as opposed to someone else).

Why is を used with 海外旅行 and not に or へ? In English we say “go to travel” or “go on a trip.”

In Japanese, you’re literally saying “do an overseas trip”:

海外旅行をする = to do an overseas trip (→ to travel abroad)

So 海外旅行 is the direct object of the verb する, and を marks that object.

Compare:

日本に行きたいです。
→ I want to go to Japan. (に marks the destination.)
日本へ行きたいです。
→ I want to go to Japan. (へ also marks direction.)
日本へ海外旅行をしたいです。
→ unnatural; you don’t travel “to” using する this way.

To talk about where you go, you usually combine:

Place + に/へ + 行く (go to a place)
Activity + を + する (do some activity)

In this sentence, the focus is on the activity of traveling abroad, not the specific country, so: 海外旅行をしたいです。

What’s the difference between 旅行 and 海外旅行 here?

旅行 = trip, travel (general; can be domestic or overseas)
海外旅行 = overseas trip, traveling abroad (specifically outside your country)

If you say:

連休には友達と一緒に旅行をしたいです。
→ I want to take a trip with my friends during the long holiday.
(Could be within Japan or abroad—unspecified.)
連休には友達と一緒に海外旅行をしたいです。
→ I want to travel abroad with my friends during the long holiday.
(Clearly outside Japan / outside your own country.)

So 海外 adds the nuance of going abroad.

How would I say “I don’t want to travel abroad with my friends during the long holiday”?

You negate the –たい form:

したい → したくない (don’t want to do)

So:

連休には友達と一緒に海外旅行をしたくないです。
→ I don’t want to travel abroad with my friends during the long holiday.

Casual version (to close friends, etc.):

連休には友達と一緒に海外旅行をしたくない。

Could I say 行きたい instead of したい? For example, 海外旅行に行きたいです?

Yes, but there’s a slight nuance difference.

海外旅行をしたいです。
→ I want to take an overseas trip / travel abroad (focus on the activity of traveling.)
海外旅行に行きたいです。
→ I want to go on an overseas trip (focus a bit more on going on the trip.)

Both are natural and often interchangeable in everyday speech.
Your original sentence could be rephrased as:

連休には友達と一緒に海外旅行に行きたいです。

What exactly does 連休 mean? How is it different from 休み?

休み (やすみ) = break, day off, holiday, vacation (very general word)
連休 (れんきゅう) = consecutive holidays, a string of days off in a row

So:

休みには旅行をします。
→ On my days off, I travel. (Could be single days off.)
連休には旅行をします。
→ During long weekends / extended holidays, I travel.
(Specifically when there are several days off in a row.)

In Japan, 連休 often refers to things like:

Golden Week
Silver Week
Long weekends where a holiday is next to a weekend

So 連休には…したいです suggests planning something a bit bigger, like an overseas trip, because you have several days off together.