Gents,
There's a 'refrigerator' of sorts being used today in various backwaters that allows vaccines to be kept cool. It consists of little but two clay pots, some sand, and some water. You need to understand thermodynamics to design one; the pots must be certain sizes in relation to each other, there needs to be X amount of sand and Y amount of water, etc., but the technique and the idea can easily be taught.
'Scientific American' had an article in the last 2 years or so on this very topic.
Gears could have been used in the 1650s or earlier for pedal bikes IF quite a number of other things were introduced along with them; cheap steel makingfor a start and screw lathes rather than hand lathes. Those two items don't require much more knowledge than gearing either.
Communications is an interesting angle to explore. Digital semaphores instead of flag/lantern semaphores could give a society fast, long distance comms well before they could make the 1000s of miles necessary for an electric telegraph. Pratchett features digital semaphores; the 'clacks', in his more recent 'Discworld' novels.
A digital semaphore would have a 3x3 arrangement of 'shutters' or 'windows' that between alternate between white and black. The towers could be tall, free-standing structures; think windmills - another nifty hi-to-lo tech gift, or simple huts built aloft on spars; much like the cranes the classical world used. With a pretty crude telescope (yet another hi-to-lo gift), one tower can read the other tower's signas at 20km or so depending on terrain. At night, lanterns are hung in which of the nine positions are required for symbol in a message.
L. Sprague de Camp's wonderful 'Ancient Engineers' is a neat place to start.
And, of course, there's always gunpowder. If they have alchemy, they can have gunpowder.
Sincerely,
Larsen