Some interesting responses:
Solar lights - like the ones for your garden that you stick in the ground. And they automatically turn on at night. Blows my mind whenever I go out to my patio at night.
A regular LCD solar calculator. You can get cheap ones for a couple bucks at a dollar store. Not only can it do complex math in a fraction of the amount of time, but it's got integrated circuits, a display, batteries, a photovoltaic cell, etc.
A laser pointer
Lasers didn't exist in 1917, that keeps escaping me. In fact, lasers didn't properly exist until 1960. It was only that very year that Albert Einstein even predicted they were possible.
A big bottle of generic antibiotics.
This $10 smart phone
While it may be a piece of shit to us, any smart phone would completely amaze anyone from 100 years ago.
A modern hybrid corn seed and the plant that grows from it. From the thirties onward corn yields increased sixfold in the US, a simple corn plant would seem like witchcraft to a farmer from 1917. Add in a Bt-toxin producing GM corn and their mind will be blown.
Even without the internet you still have:
*A screen capable of displaying moving images, all controlled by touching it
*An insanely small camera, complete with a live display
*An insanely small microphone, able to be combined with camera
*A calculator capable of calculating numbers far greater than anything possible at the time
*A digital clock
*A speaker capable of playing several different pieces of music on command, far smaller than any radio available at the time
If you have time to download anything before going back to 1917, you also can have:
*Several different interactive games
*Something like Wikipedia, complete with the sum of human knowledge, including important historical events coming up in future years for their time period.
You could probably get a lithium battery for under 10 dollars. That would definitely change history
That would actually change history even if it were released 30 years ago. IIRC, the first li-ion battery wasn't released until 1991.
Makes it all the more shocking to realize how much has changed just in the past 100 years. Someone said that, to an observer from any time before the 1930s, it would seem as if we are currently experiencing the Singularity. It's hard to wrap your mind around that when things are moving in real time and it takes several years to go from hearing about some new, potentially vaporous technology to seeing it on the market (i.e. graphene and deep learning). But 100 years ago, just experiencing the past 5 years of the present would be beyond comprehension.