I’m assuming many traditional “girl” toys have been around for ages because they tend to imitate domestic tasks (e.g. baby dolls simulate childcare). What were “boy” toys like before the the invention of monster trucks and the discovery of T-Rex?
I’m interested in the answers to this question for other parts of the world as well.
I can't say for sure it was a majority, but at least a noteworthy portion were toy soldiers.
Most of what I'm writing here is from Playing at the World by Jon Peterson. It's primarily a history of Dungeons and Dragons but that came out of a tradition of miniature war gaming, so he also gets into a bit of a history of toy soldiers:
The Weisskunig, a compendium of illustrations glorifying the early sixteenth-century Holy Roman Emperor Maximilian I, depicts in one of its plates (Ch 17, Second Part) the young emperor at play with a pair of bronze knights deployed in imagined tournaments; in 1516, he bequeathed a similar pair of miniature knights, made on commission by an armorer, to the child king of Hungary.
You can see that illustration here. The miniature knights sit on miniature horses, and play involves pushing them towards each other to dismount the opposing knight:
https://www.metmuseum.org/art/collection/search/431187
Some of the earliest tin soldiers are attributed to Johann Gottfried Hilpert in the late 18th century. He was manufacturing tableware before making soldiers, rather than a toy manufacturer adding soldiers to his his line of toys. They were very two-dimensional with a base around their feet to allow them to stand. This style of soldier would be later known as "flats."
These tin soldiers are well-known enough that Goethe references them in autobiographical work published between 1811-1814. (The first part of Aus meinem Leben: Dichtung und Wahrheit) It's in the context of describing a dream where he and a girl are playing with them. Play seems to consist of tossing small objects at the opposing forces to knock them over. (This pattern of play would appear later with Robert Louis Stevenson and H.G. Wells.) He doesn't single out whether it would be unusual for a girl to play with soldiers. He does describe the toy soldiers in the dream in contrast to what he was used to, noting they were fully-shaped instead of flat, and had no base around their feet. Which shows the toy soldiers he was used to looked a lot like the ones manufactured by Hilpert.
Another example can be seen in E. T. A. Hoffmann's The Nutcracker and the Mouse King from 1816. Marie and Fritz receive toys as Christmas presents. Marie receives dolls and a tea set. Fritz receives toy cavalry. These are described as having silver swords, and when the Nutcracker loses his sword, Fritz is convinced to provide one as a replacement. So these seem to be larger than the tin soldiers, which would have been cast as a single piece together with their weapon.
By the end of the 19th century, toy soldiers were still going strong. In 1893, English toy-maker William Britain Jr. developed a process for casting hollow full-round tin soldiers, which quickly supplanted German imports for England. By 1910, the Britains company was manufacturing over 200,000 tin soldiers per week.
H.G. Wells' Little Wars published in 1913, was a set of rules for playing with toy soldiers. This is well into trains also being a thing, but I mention it because it was subtitled a game for boys from twelve years of age to one hundred and fifty and for that more intelligent sort of girl who likes boys' games and books which would support the idea that playing with toy soldiers was widely seen as a game for boys.
Like I said, this comes from Jon Peterson who was focused on the use of miniatures in war gaming, so other types of toys that don't involve war would go overlooked in this summary.