A staple of children’s literature! Some children (or one child, alone) go to visit some relatives (often ones they either haven’t met before or don’t know well) or stay with other people (again, often ones they don’t really know) because they’re being cared for by someone temporarily because of an emergency or because they or their parents are traveling or permanently because they’re orphans. The reasons can cover quite a lot of territory, but staying with people they don’t or family they don’t usually live with can be an exciting adventure or a segue to further adventures. Sometimes the house where they’re staying is somehow strange or the people are or both, and the children uncover buried secrets, old mysteries, ghosts, magic, etc. or just have some zany fun! The possibilities are endless!
For books on a somewhat similar theme, see Children Go to Live with Strangers.
For more adventures that occur away from home and in strange places – see also:
It’s Been a Long Strange Field Trip
How I Spent My Summer Vacation
Getting There is Half the Fun
Sometimes, the most exciting things happen on the way somewhere!
The Case of the Vanishing Villain
Flee Jay and Clarice are taking a ferry across Lake Michigan by themselves to visit their grandparents when they hear that an escaped convict might try to stow away on the ferry. At first, they doubt that the convict could have sneaked aboard, but then a woman screams that there was a strange man hiding in her room. The convict is on the boat, but where?

Mailing May (1997)
A young girl in 1914 wants to go visit her grandmother, but her family can’t afford the train ticket . . . until they realize that they can afford for her to travel as mail. Based on a true story. By Michael O. Tunnell.
Time Travel

When Sally goes to stay with an aunt she’s never met before, she finds a portrait of a young girl who looks very much like her wearing old-fashioned clothes and holding a doll. This young girl, also named Sally, used to live in her aunt’s house many years ago, and Sally begins to have visions of her life there and how she lost her beloved doll, Elizabeth, who is a little bit magic and is still in the house, waiting to be found. Part mystery, part fantasy.

The Time Garden (1958)
While visiting a relative during the summer, the children discover something strange about her thyme garden. It turns out that it’s really a time garden, inhabited by the mysterious Natterjack. When the children pick a spring of thyme, the variety of thyme they choose takes them to another time.
Tom’s Midnight Garden (1958)
Tom is spending the summer with his aunt and uncle. He is lonely at first, but he discovers that when the clock chimes thirteen times at night, he can go back in time, where he befriends an equally lonely girl named Hatty. By Phillipa Pearce.
Series

Particularly the first one, where four siblings are staying with a great aunt in Scotland and travel to different periods in time by using a magical key in the keep of an old castle.
A girl and her aunt travel to various periods of time using a time machine that was left to them by a distant relative.
Travel to Magical Lands
Knight’s Castle (1956)
A group of cousins has magical adventures with a toy castle that comes to life.
Series
A children’s classic! Full of religious allegory. Particularly,he first book, The Lion, the Witch, and the Wardrobe where four siblings who are sent to stay at a house in the country during World War II find a magical wardrobe that allows them to cross over into the magical land of Narnia.
Fun Adventures

Twins Randi and Sandi go to visit their cousin Mandy. The three girls look alike, and when Mandy tells them about trying out for a part in a commercial, they consider a scheme that will give Mandy an edge and help her get back at a rival for the part.
A Long Way from Chicago and A Year Down Yonder
The adventures of a brother and sister who stay with their eccentric grandmother in rural Illinois during and just after the Great Depression.
Series
The hilarious escapades of Cathy and her crazy cousin Courtney in New York and California. By Judi Miller. 1993-1997.
Ghost Stories
The dolls in an old dollhouse seem to be reenacting the unsolved murder of Amy’s grandparents years ago.

A young woman and her young niece go to visit the niece’s maternal grandmother and discover the disturbing circumstances of the death of the niece’s other aunt, Emily. Emily’s restless, spiteful, and selfish spirit resides in a garden gazing globe, reaching out to touch her vulnerable young niece, Jane.
The Mystery of the Haunted Trail
When Brian visits his pen pal in Hawaii for the summer, he confronts a ghostly mystery based on Hawaiian legends. By Janet Lorimer.
Mysteries

Laura and Bill are visiting their Uncle Joe, who is the caretaker of a large hotel in New Hampshire. The hotel is closed for the winter, but the children investigate the ghost who supposedly haunts it and may have their uncle under her spell. By Janet Adele Bloss.
Meg Mackintosh and the Case of the Missing Babe Ruth Baseball
Years ago, Meg’s grandfather’s sister hid his prized baseball, autographed by Babe Ruth. She never told him what she did with it, but she left clues in the style of a treasure hunt to help him find it. Can Meg use her knowledge of nursery rhymes to solve the puzzle for her grandfather?

Mystery of the Angry Idol (1965)
When Jan’s family moves overseas for her father’s work, she stays in the United States to go to school, living with her mysterious, reclusive great-grandmother in an old house filled with Asian art and secrets. By Phyllis A. Whitney.
A girl who has gone to live with her father in Turkey, who has been staying in the house of some friends, becomes friends with a troubled orphan girl and investigates strange things happening in the house where they are staying. By Phyllis Whitney.

Four cousins search for a hidden treasure in their grandfather’s mansion during a blizzard.
A girl and her family go to visit a relative in South Africa and solve the mystery of a theft that occurred years ago. By Phyllis Whitney.
The Secrets of the Pirate Inn (1968)
Three children help an old man to search for a hidden treasure in an old pirate’s inn. Disney called it Secrets of the Pirate’s Inn for the made-for-tv movie, and there are some notable differences between the book and the movie. By Wylly Folk St. John.

Something Queer in the Wild West
Gwen and Jill investigate a haunted barn on Gwen’s uncle’s ranch in New Mexico.
A young deaf girl is kidnapped for ransom. Her best friend searches for her while she struggles to leave clues to help her friend learn her kidnapper’s identity.
A girl staying with her grandmother at her bed-and-breakfast investigates the theft of letters written by Mark Twain.
Series
Two pairs of fraternal twins have adventures and solve mysteries. Early books were more adventure/general fiction than mystery, but the series evolved over time. Books in this series are public domain and are available on Project Gutenberg. A Stratemeyer Syndicate series. 1904-1992.
Four children who lived alone in a boxcar after the death of their parents are taken in by their grandfather and solve mysteries everywhere they go. 1924, 1942-Present.
About three siblings who solve mysteries together. Many of them are treasure hunts. They frequently occur when the children are visiting with their grandparents. By Peggy Parish. 1966-1986.
A classic mystery series featuring a girl and her group of friends who solve mysteries and support good causes. There are a few books in the series which would qualify for this theme. Created by Julie Campbell and continued by other authors. 1948-1986.
New Perspectives on Life
Nothing magical, supernatural, scary, or mysterious. Just, staying with different, perhaps unusual, people can give people a new perception of themselves and their lives.

A spoiled little princess learns about friendship and hidden depths and finds her own inner beauty when she is sent to live with a family of commoners for a time.
Thunder Cake (1990)
A girl talks about how her grandmother, who she likes to call “Babushka” because she originally came from Russia, cured her of her fear of storms by teaching her to make a special kind of cake, Thunder Cake. By Patricia Polacco.