What is a mission trip? What does it mean when a person says "I have to go on a mission trip for a week", and the mission trip destination is in the United States of America, the person's home country?

I have the impression that a mission trip can actually be a missionary activity. Yet, I also have the impression that a mission trip is just a fun excursion or camping trip.
A mission trip should always include missionary activity.  The activity itself is widely varied and does not require international travel: it can include volunteer work, evangelism, teaching, or a number of other things.  The words &quot;mission trip&quot; speak to the purpose of the trip, not the activity of the trip.  The mission of a mission trip is to, either directly or indirectly, lead people toward God.
Oftentimes, these trips are exciting.  A person's motive for going on a mission trip, as with all things, is complex.  One of my pastor friends calls secondary motives, which everyone has to some degree, a &quot;shadow mission&quot;.  For example, say there is a mission trip planned to build a house in Mexico with Casas Por Cristo.  The mission is to build a house for a poor family in the name of Jesus Christ, with the hope that the family receiving the house and people in that area would be positively affected by Christianity.  Casas Por Cristo works with local pastors to evangelize to the area; the people on the mission trip simply provide the labor (and money) to build the house itself.  A shadow mission for someone attending the trip may be to experience international travel, or spend time with a cute girl also on the trip, or to appear spiritual, or to have fun.
I only mention the idea of a &quot;shadow mission&quot; to explain why you may get the impression that it is &quot;just a fun excursion or camping trip.&quot;  When that pastor would take college students on mission trips, he would urge them to evaluate their own shadow missions and to be aware that the true mission is leading people toward God.
Mission trips are generally short-term events.  Most trips are two weeks long or less, though trips can certainly be longer, up to a year or so.  Anything that is a year or longer is usually categorized as long-term missions, performed by a missionary.
Here are examples of mission trips:

Spending a week in Mexico building a house
Repairing a church building in Honduras
Working in a Russian orphanage
Smuggling Bibles into China or other closed countries
Helping clean up after Hurricane Sandy
Spending a week in San Francisco handing out gospel tracts and talking to people about God
Traveling 2 hours to a youth camp to repair/build cabins

Here are examples of things that are not mission trips.

Going to a Christian concert
Attending a Christian summer camp
Going camping with the Youth Group at your church
Driving to Disney World with a church group to ride rollercoasters
Watching a Christian movie at a theater with other Christians

That said, it is common in a one or two-week mission trip to include a fun day on the way home (which may be going to Disney World, seeing a concert, going to the beach, etc.).