Actually, That's My Assistant: In the Emily books and a few other stories, characters will be expecting the visit of a famous authoress. The doorbell will ring, and they will meet a tall, elegant grand dame with a serene countenance... and a short, homely-looking bustling matron. They always think the grand dame is the writer, but...
Arcadia: Prince Edward Island, when you're not roaming its Ghibli Hills. Cheerful farmers who work in harmony with the land are a frequent appearance.
Baby-Doll Baby: In Magic for Marigold, a Lesley family heirloom is a life-size wax doll of a baby that some long-deceased family member had made of her own baby, who died. The mother not only dressed the doll in her baby's clothes, but carried it with her, talked to it, and slept with it as if it were real.
Badass Grandma: Appear rather frequently, most notable example is Old Grandmother in Magic for Marigold.
Cute Mute: Kilmeny, from Kilmeny of the Orchard, has been secluded and sheltered for all of her life, and retains a childlike simplicity.
Deconfirmed Bachelor: Uncle "Klondike" Lesley fits every box on the list. Despite his whole family trying to set him up with a lady friend, he finds fault with every woman that they choose; he scorns the idea of love, especially Love at First Sight (although he mentions having witnessed a few passionate love affairs), he swears he'll remain a bachelor to the end of his days — then falls head-over-heels in love with a female pediatrician that he asks to look after his dying niece.
Dry Crusader: A few of her stories and novels include characters who champion for Temperance and alcohol prohibition, a very timely subject. Only "The Deacon's Painkiller," a short story about a particularly self-righteous minister, features the crusader getting karmically rebuked.
The Edwardian Era: Notable in the Emily books, when Emily meets a slightly loopy woman who happily tells the story of "The day I spanked the king" (read: Edward.)
Feminine Women Can Cook: Almost all of her heroines have some talent at housekeeping and cooking. Justified, as they are all of a social class and era when cooking would be in a woman's basic skill set. Frequently, Montgomery puts cooking and domesticity in a positive, even powerful light. For heroines like Valancy and Pat, a woman in charge of her own house and her own kitchen is a woman independent and creative. A notable exception is the Story Girl from the novel of the same name and The Golden Road.
Jane in particular takes to cooking like a duck to water, feeding both herself and her father, even though she was never allowed to cook before. She does prudently buy a cookbook first, and donuts defeat her.
The Gay Nineties: Although the characters aren't exactly off to live the "gay" life, for the most part. Earlier stories of hers have Queen Victoria cited as the reigning monarch, and several times characters express respect for her.
Ghibli Hills: Prince Edward Island, from the red dust of its roads to the gentle moaning of the sea, which is never far off.
Green-Eyed Monster: Montgomery frequently revisits the character of the widowed mother who is fanatically attached to her only begotten son, and fiercely jealous of any romantic attachments in his life.
Growing Up Sucks: This trope hits Marigold Lesley hardest. She completely loses her ability to imagine Sylvia, her dear imaginary friend.
Have a Gay Old Time: Like any other writer of her era, Montgomery runs hard into this trope. She loved the use of the term "puss" to describe a cat, and pretty much every heroine of hers is called "queer" sooner or later — meaning, unusual, defiant of social custom, difficult to define.
Hysterical Woman: Pat of Silver Bush, in her first book, is a nascent version of this trope. She loves very, very deeply and tends to take change badly — it takes her entire family to calm her down after her father shaves his mustache. As a young girl she is already gaining a reputation for going into hysterics; as she grows up she has slightly better control of her emotions, but is always considered odd.
Imaginary Friend: The more isolated characters have these, including Anne before she is adopted, Emily and Marigold (whose father also had imaginary friends).
Love at First Sight: A generous helping — although frequently this love is more like "a flash of recognition between kindred spirits," which can later blossom into love. Klondike Lesley's instant love for Dr. M. Woodruff Richards is a prominent example.
New Technology Is Evil: A fairly common trope in Montgomery's work. The Murrays in the Emily series are considered old-fashioned by their neighbors for using candles instead of kerosene lamps, and in "Pat of Silver Bush" makes fun of her cousins for getting their water from a tap.
The trope is downplayed with and lampshaded by Anne in "Anne of Avonlea" when she acknowledges her misgivings about telephones are probably sentimental and unrealisitc.
Interestingly, Montgomery's ambivalence about new technology did not extend to cars, which are portrayed very positively in later works such as "The Blue Castle" and "Jane of Lantern Hill."
Old Maid: the heroine of "The Materialization of Duncan Mc Tavish,"
Orphan's Ordeal: Anne. Emily. For starters. And there's a whole book of short stories (Akin to Anne) dedicated to nothing but orphan's ordeals.
The Penance: In one short story, a high-living, flippant young woman is convinced that she caused the death of her sister, a respected and hard-working pillar of the community. The young woman trades her dancing for Bible reading, her pretty clothes for charitable work — and the less she likes it, the more she does it. She plans on repenting so for the rest of her life.
Put on a Bus: One of the most often repeated phrases in Montgomery's works is probably, "He/she/they left for the Prairies." During the Edwardian and Victorian Eras, The Government of Canada ran a truly massive campaign to recruit people to settle the Prairie Provinces. The Prairies were far enough for characters to disappear into—and close enough (thanks to the CN Railroad) that they can come back at any time.
The Promise: Treated very darkly in Kilmeny of the Orchard. Kilmeny's aunt and uncle fervently believe her muteness is owed to a vow of silence taken by her mother during her pregnancy, to never speak another word to the father that rejected her. Even when her father lay dying and asked his daughter to speak one word of reconciliation, Kilmeny's mother refused. She didn't speak a word until after her baby's birth, when she broke down sobbing.
Race for Your Love: "Here Comes the Bride." This is what happens when a headstrong young lady, sick of the gossip around her, says she'll just marry the next man who asks her... and then a gossip takes it to the town.
Rebellious Princess: Varvara, a Russian princess, runs away from her aunt and spends an afternoon playing with Marigold. And Marigold doesn't believe Varvara when she says she is a princess.
Self-Made Man: "Klondike Lesley" is the most prominent example — as his nickname suggests, he struck gold in the Klondike and returned home after traveling the world, to be endlessly fussed over and matchmade by his female relatives. Other Self Made Men appear in the short stories.
The Storyteller: The Story Girl, true to her name, is the most prominent, to the point that most people forget her real name is Sarah. Emily comes quite close, though.
Victorious Childhood Friend: Happens perhaps on an even balance with Love at First Sight. Most of Montgomery's characters live in small towns, so most of the people of the community have grown up together, and marry people they've known since their schooldays.