One of these things I've been thinking about (having the MC introduce in a historical reincarnation/isekai-style ongoing story I've been working on), is the
Sailing Hydrofoil, or 'Hydrosail'. Only really developed and introduced since the 1980s IOTL; but imagine the possibilities if they got introduced a couple, or even several centuries earlier, during or even preceding the 'Age of Sail' (allowing these vessels to achieve speeds exceeding double the wind speed). And it'd be a damn sight easier, lower-tech and more plausible, as a first step on the foil technology ladder, than the use of aerofoils to achieve powered flight, as wound up being the case IOTL.
And if one thinks about it, there's very little in the way of technological obstacles to prevent the development/introduction of hydrofoils at any stage of history where woodworking's a thing; with the Chinese 'bamboo dragonfly' and the Aboriginal Australian boomerang already utilising the pre-requisite basal level of foil technology.
For instance, one can easily imagine an ATL where some random person from a coastal Aboriginal tribe, or even paleolithic Europe, came up with the idea of strapping a set of boomerangs beneath his sailing canoe to provide lift in the same manner, and developed the first hydro-sail several millennia ago (potentially even over 10kya). If you could achieve sailing speeds of 30 knots+ that far back (vastly shortening transit times, to be on a par with or quicker than those we only managed to attain at the dawn of the 20th century IOTL), how much more viable would historical trans-oceanic exploration and merchant journeys have been? And how much more metropolitan might the world have been as a result, so much earlier?